mountd: minor v4root_set cleanup, check strdup return

Move more of v4root_set into a helper function.

Also, check the return value from strdup.  (We don't really handle the
error well yet--we'll end up giving negative replies to export upcalls
when we should be giving the kernel exports, resulting in spurious
-ENOENTs or -ESTALE's--but that's better than crashing with a NULL
dereference.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>