vfs: Use file_cred for VOP_DEALLOCATE in vn_deallocate if non-NULL

This changes vn_deallocate() to match the behavior of vn_rdwr() when
picking which ucred to use. That is, vn_deallocate() uses file_cred for
making VOP call if it is non-NULL, or use active_cred otherwise.

Sponsored by: The FreeBSD Foundation
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31712