vfs_default: Change vop_stddeallocate() from static to global

A future commit to the NFS client uses vop_stddeallocate() for
cases where the NFS server does not support a Deallocate operation.
Change vop_stddeallocate() from static to global so that it can
be called by the NFS client.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31640