swap_dev_info() does not require Giant, so Giant locking around
the loop in linprocfs_doswaps() is useless.
List of the registered filesystems is protected by vfsconf_sx,
not by the Giant. Adjust linprocfs_dofilesystems() correspondingly.

Approved by: re (delphij), des (linprocfs maintainer)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week