NFSD: Relocate the fh_want_write() and fh_drop_write() helpers

Clean up: these helpers are part of the NFSD file handle API.
Relocate them to fs/nfsd/nfsfh.h.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>