nfsd4: clean up access_valid, deny_valid checks.

Document these checks a little better and inline, as suggested by Neil
Brown (note both functions have two callers).  Remove an obviously bogus
check while we're there (checking whether unsigned value is negative).

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Neil Brown <neilb@suse.de>