Add descrip_check_write_mp() helper

... which verifies that given file table does not have file descriptors
referencing vnodes on the specified mount point. It is up to the caller
to ensure that the check is not racy.

Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37896