ocfs2: ac->ac_allow_chain_relink=0 won't disable group relink

ocfs2_block_group_alloc_discontig() disables chain relink by setting
ac->ac_allow_chain_relink = 0 because it grabs clusters from multiple
cluster groups.

It doesn't keep the credits for all chain relink,but
ocfs2_claim_suballoc_bits overrides this in this call trace:
ocfs2_block_group_claim_bits()->ocfs2_claim_clusters()->
__ocfs2_claim_clusters()->ocfs2_claim_suballoc_bits()
ocfs2_claim_suballoc_bits set ac->ac_allow_chain_relink = 1; then call
ocfs2_search_chain() one time and disable it again, and then we run out
of credits.

Fix is to allow relink by default and disable it in
ocfs2_block_group_alloc_discontig.

Without this patch, End-users will run into a crash due to run out of
credits, backtrace like this:

RIP: 0010:[<ffffffffa0808b14>] [<ffffffffa0808b14>]
jbd2_journal_dirty_metadata+0x164/0x170 [jbd2]
RSP: 0018:ffff8801b919b5b8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88022139ddc0 RCX: ffff880159f652d0
RDX: ffff880178aa3000 RSI: ffff880159f652d0 RDI: ffff880087f09bf8
RBP: ffff8801b919b5e8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000001e00 R11: 00000000000150b0 R12: ffff880159f652d0
R13: ffff8801a0cae908 R14: ffff880087f09bf8 R15: ffff88018d177800
FS: 00007fc9b0b6b6e0(0000) GS:ffff88022fd40000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 000000000040819c CR3: 0000000184017000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process dd (pid: 9945, threadinfo ffff8801b919a000, task ffff880149a264c0)
Call Trace:
ocfs2_journal_dirty+0x2f/0x70 [ocfs2]
ocfs2_relink_block_group+0x111/0x480 [ocfs2]
ocfs2_search_chain+0x455/0x9a0 [ocfs2]
...

Signed-off-by: Xiaowei.Hu <[e-mail]>
Reviewed-by: Srinivas Eeda <[e-mail]>
Cc: Mark Fasheh <[e-mail]>
Cc: Joel Becker <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ocfs2/suballoc.c

···
 	 * cluster groups will be staying in cache for the duration of
 	 * this operation.
 	 */
-	ac->ac_allow_chain_relink = 0;
+	ac->ac_disable_chain_relink = 1;
 
 	/* Claim the first region */
 	status = ocfs2_block_group_claim_bits(osb, handle, ac, min_bits,
···
 	 * Do this *after* figuring out how many bits we're taking out
 	 * of our target group.
 	 */
-	if (ac->ac_allow_chain_relink &&
+	if (!ac->ac_disable_chain_relink &&
 	    (prev_group_bh) &&
 	    (ocfs2_block_group_reasonably_empty(bg, res->sr_bits))) {
 		status = ocfs2_relink_block_group(handle, alloc_inode,
···
 
 	victim = ocfs2_find_victim_chain(cl);
 	ac->ac_chain = victim;
-	ac->ac_allow_chain_relink = 1;
 
 	status = ocfs2_search_chain(ac, handle, bits_wanted, min_bits,
 				    res, &bits_left);
···
 	 * searching each chain in order. Don't allow chain relinking
 	 * because we only calculate enough journal credits for one
 	 * relink per alloc. */
-	ac->ac_allow_chain_relink = 0;
+	ac->ac_disable_chain_relink = 1;
 	for (i = 0; i < le16_to_cpu(cl->cl_next_free_rec); i ++) {
 		if (i == victim)
 			continue;

fs/ocfs2/suballoc.h

···
 
 	/* these are used by the chain search */
 	u16    ac_chain;
-	int    ac_allow_chain_relink;
+	int    ac_disable_chain_relink;
 	group_search_t *ac_group_search;
 
 	u64    ac_last_group;