Fix lockdep circular locking false positive involving sa_lock

There are two different deadlock scenarios, but they share a common
link, which is
thread 1 holding sa_lock and trying to get zap->zap_rwlock:
    zap_lockdir_impl+0x858/0x16c0 [zfs]
    zap_lockdir+0xd2/0x100 [zfs]
    zap_lookup_norm+0x7f/0x100 [zfs]
    zap_lookup+0x12/0x20 [zfs]
    sa_setup+0x902/0x1380 [zfs]
    zfsvfs_init+0x3d6/0xb20 [zfs]
    zfsvfs_create+0x5dd/0x900 [zfs]
    zfs_domount+0xa3/0xe20 [zfs]

and thread 2 trying to get sa_lock, either in sa_setup:
   sa_setup+0x742/0x1380 [zfs]
   zfsvfs_init+0x3d6/0xb20 [zfs]
   zfsvfs_create+0x5dd/0x900 [zfs]
   zfs_domount+0xa3/0xe20 [zfs]
or in sa_build_index:
   sa_build_index+0x13d/0x790 [zfs]
   sa_handle_get_from_db+0x368/0x500 [zfs]
   zfs_znode_sa_init.isra.0+0x24b/0x330 [zfs]
   zfs_znode_alloc+0x3da/0x1a40 [zfs]
   zfs_zget+0x39a/0x6e0 [zfs]
   zfs_root+0x101/0x160 [zfs]
   zfs_domount+0x91f/0xea0 [zfs]

From there, there are different locking paths back to something
holding zap->zap_rwlock.

The deadlock scenarios involve multiple different ZFS filesystems
being mounted.  sa_lock is common to these scenarios, and the sa
struct involved is private to a mount.  Therefore, these must be
referring to different sa_lock instances and these deadlocks can't
occur in practice.

The fix, from Brian Behlendorf, is to remove sa_lock from lockdep
coverage by initializing it with MUTEX_NOLOCKDEP.

Signed-off-by: Jeff Dike <[email protected]>

module/zfs/sa.c

@@ -1014,7 +1014,7 @@ sa_setup(objset_t *os, uint64_t sa_obj, sa_attr_reg_t *reg_attrs, int count,
 	}
 
 	sa = kmem_zalloc(sizeof (sa_os_t), KM_SLEEP);
-	mutex_init(&sa->sa_lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&sa->sa_lock, NULL, MUTEX_NOLOCKDEP, NULL);
 	sa->sa_master_obj = sa_obj;
 
 	os->os_sa = sa;