fs: don't use igrab() while holding i_lock

Fix the incorrect use of igrab() inside the i_lock in NFS and Ceph‥

If we are already holding the i_lock, we have a reference to the
inode so we can safely use ihold() to gain an extra reference. This
avoids hangs due to lock recursion on the i_lock now that the
inode_lock is gone and igrab() uses the i_lock itself.

Signed-off-by: Dave Chinner <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: [e-mail]
Cc: Ryan Mallon <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ceph/addr.c

···
 		ci->i_head_snapc = ceph_get_snap_context(snapc);
 	++ci->i_wrbuffer_ref_head;
 	if (ci->i_wrbuffer_ref == 0)
-		igrab(inode);
+		ihold(inode);
 	++ci->i_wrbuffer_ref;
 	dout("%p set_page_dirty %p idx %lu head %d/%d -> %d/%d "
 	     "snapc %p seq %lld (%d snaps)\n",

fs/ceph/snap.c

···
 
 		dout("queue_cap_snap %p cap_snap %p queuing under %p\n", inode,
 		     capsnap, snapc);
-		igrab(inode);
-		
+		ihold(inode);
+
 		atomic_set(&capsnap->nref, 1);
 		capsnap->ci = ci;
 		INIT_LIST_HEAD(&capsnap->ci_item);

fs/nfs/nfs4state.c

···
 		state->owner = owner;
 		atomic_inc(&owner->so_count);
 		list_add(&state->inode_states, &nfsi->open_states);
-		state->inode = igrab(inode);
+		ihold(inode);
+		state->inode = inode;
 		spin_unlock(&inode->i_lock);
 		/* Note: The reclaim code dictates that we add stateless
 		 * and read-only stateids to the end of the list */