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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jeff Dike <jdike@akamai.com>
Closes #9110