Linux: g/c leftover fence in zfs_znode_alloc

The port removed provisions for zfs_znode_move but the cleanup missed
this bit. To quote the original:

[snip]
    list_insert_tail(&zfsvfs->z_all_znodes, zp);
    membar_producer();
    /*
     * Everything else must be valid before assigning z_zfsvfs makes the
     * znode eligible for zfs_znode_move().
     */
    zp->z_zfsvfs = zfsvfs;
[/snip]

In the current code it is immediately followed by unlock which issues
the same fence, thus plays no role in correctness.

Reviewed-by: Matt Macy <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Mateusz Guzik <[email protected]>
Closes #11115

Author: mjguzik

module/os/linux/zfs/zfs_znode.c

@@ -620,7 +620,6 @@ zfs_znode_alloc(zfsvfs_t *zfsvfs, dmu_buf_t *db, int blksz,
 	mutex_enter(&zfsvfs->z_znodes_lock);
 	list_insert_tail(&zfsvfs->z_all_znodes, zp);
 	zfsvfs->z_nr_znodes++;
-	membar_producer();
 	mutex_exit(&zfsvfs->z_znodes_lock);
 
 	unlock_new_inode(ip);