fd: make serialization in fdescfree_fds conditional on hold count

p_fd nullification in fdescfree serializes against new threads transitioning
the count 1 -> 2, meaning that fdescfree_fds observing the count of 1 can
safely assume there is nobody else using the table. Losing the race and
observing > 1 is harmless.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D27522