deadlock between spa_errlog_lock and dp_config_rwlock

There is a lock order inversion deadlock between `spa_errlog_lock` and
`dp_config_rwlock`:

A thread in `spa_delete_dataset_errlog()` is running from a sync task.
It is holding the `dp_config_rwlock` for writer (see
`dsl_sync_task_sync()`), and waiting for the `spa_errlog_lock`.

A thread in `dsl_pool_config_enter()` is holding the `spa_errlog_lock`
(see `spa_get_errlog_size()`) and waiting for the `dp_config_rwlock` (as
reader).

Note that this was introduced by #12812.

This commit address this by defining the lock ordering to be
dp_config_rwlock first, then spa_errlog_lock / spa_errlist_lock.
spa_get_errlog() and spa_get_errlog_size() can acquire the locks in this
order, and then process_error_block() and get_head_and_birth_txg() can
verify that the dp_config_rwlock is already held.

Additionally, a buffer overrun in `spa_get_errlog()` is corrected.  Many
code paths didn't check if `*count` got to zero, instead continuing to
overwrite past the beginning of the userspace buffer at `uaddr`.

Tested by having some errors in the pool (via `zinject -t data
/path/to/file`), one thread running `zpool iostat 0.001`, and another
thread runs `zfs destroy` (in a loop, although it hits the first time).
This reproduces the problem easily without the fix, and works with the
fix.

Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #14239
Closes #14289

module/zfs/zfs_ioctl.c

@@ -5695,17 +5695,12 @@ zfs_ioc_error_log(zfs_cmd_t *zc)
 {
 	spa_t *spa;
 	int error;
-	uint64_t count = zc->zc_nvlist_dst_size;
 
 	if ((error = spa_open(zc->zc_name, &spa, FTAG)) != 0)
 		return (error);
 
 	error = spa_get_errlog(spa, (void *)(uintptr_t)zc->zc_nvlist_dst,
-	    &count);
-	if (error == 0)
-		zc->zc_nvlist_dst_size = count;
-	else
-		zc->zc_nvlist_dst_size = spa_get_errlog_size(spa);
+	    &zc->zc_nvlist_dst_size);
 
 	spa_close(spa, FTAG);