nfsd4: idmap upcalls should use unsigned uid and gid

We shouldn't be using negative uid's and gid's in the idmap upcalls.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>