ufs/suspend: deny suspension if the calling process has a file from mp opened for write

Also deny suspension if we cannot check the above condition race-free
because there is more than one thread in the calling process.

PR: 267628, 267630
Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37896