Remove zl_issuer_lock from zil_suspend().

This locking was recently added as part of openzfs#14979. But appears it
is illegal to take zl_issuer_lock while holding dp_config_rwlock,
taken by dsl_pool_hold().  It causes deadlock with sync thread in
spa_sync_upgrades().  On a second thought, we should not
need this locking, since zil_commit_impl() we call below takes
zl_issuer_lock, that should sufficiently protect zl_suspend reads,
combined with other logic from openzfs#14979.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#15103

module/zfs/zil.c

@@ -3941,13 +3941,11 @@ zil_suspend(const char *osname, void **cookiep)
 		return (error);
 	zilog = dmu_objset_zil(os);
 
-	mutex_enter(&zilog->zl_issuer_lock);
 	mutex_enter(&zilog->zl_lock);
 	zh = zilog->zl_header;
 
 	if (zh->zh_flags & ZIL_REPLAY_NEEDED) {		/* unplayed log */
 		mutex_exit(&zilog->zl_lock);
-		mutex_exit(&zilog->zl_issuer_lock);
 		dmu_objset_rele(os, suspend_tag);
 		return (SET_ERROR(EBUSY));
 	}
@@ -3961,7 +3959,6 @@ zil_suspend(const char *osname, void **cookiep)
 	if (cookiep == NULL && !zilog->zl_suspending &&
 	    (zilog->zl_suspend > 0 || BP_IS_HOLE(&zh->zh_log))) {
 		mutex_exit(&zilog->zl_lock);
-		mutex_exit(&zilog->zl_issuer_lock);
 		dmu_objset_rele(os, suspend_tag);
 		return (0);
 	}
@@ -3970,7 +3967,6 @@ zil_suspend(const char *osname, void **cookiep)
 	dsl_pool_rele(dmu_objset_pool(os), suspend_tag);
 
 	zilog->zl_suspend++;
-	mutex_exit(&zilog->zl_issuer_lock);
 
 	if (zilog->zl_suspend > 1) {
 		/*