fs: jfs: fix possible NULL pointer dereference in dbFree()

In our fault-injection testing, the variable "nblocks" in dbFree() can be
zero when kmalloc_array() fails in dtSearch(). In this case, the variable
"mp" in dbFree() would be NULL and then it is dereferenced in
"write_metapage(mp)".

The failure log is listed as follows:

[ 13.824137] BUG: kernel NULL pointer dereference, address: 0000000000000020
...
[ 13.827416] RIP: 0010:dbFree+0x5f7/0x910 [jfs]
[ 13.834341] Call Trace:
[ 13.834540] <TASK>
[ 13.834713] txFreeMap+0x7b4/0xb10 [jfs]
[ 13.835038] txUpdateMap+0x311/0x650 [jfs]
[ 13.835375] jfs_lazycommit+0x5f2/0xc70 [jfs]
[ 13.835726] ? sched_dynamic_update+0x1b0/0x1b0
[ 13.836092] kthread+0x3c2/0x4a0
[ 13.836355] ? txLockFree+0x160/0x160 [jfs]
[ 13.836763] ? kthread_unuse_mm+0x160/0x160
[ 13.837106] ret_from_fork+0x1f/0x30
[ 13.837402] </TASK>
...

This patch adds a NULL check of "mp" before "write_metapage(mp)" is called.

Reported-by: TOTE Robot <[e-mail]>
Signed-off-by: Zixuan Fu <[e-mail]>
Signed-off-by: Dave Kleikamp <[e-mail]>

fs/jfs/jfs_dmap.c

···
 	}
 
 	/* write the last buffer. */
-	write_metapage(mp);
+	if (mp)
+		write_metapage(mp);
 
 	IREAD_UNLOCK(ipbmap);