Add cond_resched to zfs_zget to prevent infinite loop

It's been reported that threads would loop infinitely inside zfs_zget. The
speculated cause for this is that if an inode is marked for evict, zfs_zget
would see that and loop. However, if the looping thread doesn't yield, the
inode may not have a chance to finish evict, thus causing a infinite loop.

This patch solve this issue by add cond_resched to zfs_zget, making the
looping thread to yield when needed.

Tested-by: jlavoy <jalavoy@gmail.com>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3349

module/zfs/zfs_znode.c

@@ -950,6 +950,8 @@ zfs_zget(zfs_sb_t *zsb, uint64_t obj_num, znode_t **zpp)
 				mutex_exit(&zp->z_lock);
 				sa_buf_rele(db, NULL);
 				ZFS_OBJ_HOLD_EXIT(zsb, obj_num);
+				/* inode might need this to finish evict */
+				cond_resched();
 				goto again;
 			}
 			*zpp = zp;