Fix deadlock between zfs umount & snapentry_expire

zfs umount -> zfsctl_destroy() takes zfs_snapshot_lock WRITER &
calls zfsctl_snapshot_unmount_cancel() which waits for
snapentry_expire() if present(only when snap is automounted).
This snapentry_expire() itself then waits for zfs_snapshot_lock
READER, resulting in a deadlock.

Fix is to take READER in zfsctl_destroy() & upgrade to WRITER
after call to zfsctl_snapshot_unmount_cancel() before calling
zfsctl_snapshot_remove(). This allows snapentry_expire() to
progress too with the READER.

Signed-off-by: Rohan Puri <rohan.puri15@gmail.com>

module/zfs/zfs_ctldir.c

@@ -566,10 +566,14 @@ zfsctl_destroy(zfsvfs_t *zfsvfs)
 		spa_t *spa = zfsvfs->z_os->os_spa;
 		uint64_t objsetid = dmu_objset_id(zfsvfs->z_os);
 
-		rw_enter(&zfs_snapshot_lock, RW_WRITER);
+		rw_enter(&zfs_snapshot_lock, RW_READER);
 		if ((se = zfsctl_snapshot_find_by_objsetid(spa, objsetid))
 		    != NULL) {
 			zfsctl_snapshot_unmount_cancel(se);
+			if (!rw_tryupgrade(&zfs_snapshot_lock)) {
+				rw_exit(&zfs_snapshot_lock);
+				rw_enter(&zfs_snapshot_lock, RW_WRITER);
+			}
 			zfsctl_snapshot_remove(se);
 			zfsctl_snapshot_rele(se);
 		}