mountd: make local functions in v4root.c static

Clean up.  set_pseudofs_security() and pseudofs_update() have no
call sites outside of v4root.c, and there are no header declarations
for either function.  Define both as static.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>