Fix file descriptor leak on pool import.

Fix file descriptor leak on pool import.

Descriptor leak can be easily reproduced by doing:

	# zpool import tank
	# sysctl kern.openfiles
	# zpool export tank; zpool import tank
	# sysctl kern.openfiles

We were leaking four file descriptors on every import.

Similar leak most likely existed when using file-based VDEVs.

External-issue: https://reviews.freebsd.org/D43529
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #15630