Fix deadlock on I/O errors during device removal

spa_vdev_remove_thread() should not hold svr_lock while loading a
metaslab. It may block ZIO threads, required to handle metaslab
loading, at least in case of read errors causing recovery writes.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #17145