audit: plug FILEDESC_LOCK leak in audit_canon_path.

MFC after:	3 days
MFV 263436-263438:

  3947 zpool(1M) references nonexistent zfs-features(5)
  4540 zpool(1M) man page doesn't describe "readonly" property
  3948 zfs sync=default is not accepted
  4611 zfs(1M) still mentions 'send -r' in synopsis
  4415 zpool(1M) man page missing "import -m" description
  4570 Document dedupditto pool property
  4572 Dedup-related documentation additions for zpool and zdb.
  1371 Add -D option description to zpool(1M) manpage
  4571 Add documentation for -T and interval to "zpool list"

MFC after:	2 weeks
Take filedesc lock only for reading when allocating new fdtable.

Code populating the table does this already.

MFC after:	1 week