Modify release_page() to handle a missing fault page.

r353890 introduced a case where we may call release_page() with
fs.m == NULL, since the fault handler may now lock the vnode prior
to allocating a page for a page-in.

Reported by: jhb
Reviewed by: kib
MFC with: r353890
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22120