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