nfsd: distinguish functions of NFSD_MAY_* flags

Most of the NFSD_MAY_* flags actually request permissions, but over the
years we've accreted a few that modify the behavior of the permission or
open code in other ways.

Distinguish the two cases a little more.  In particular, allow the
shortcut at the start of nfsd_permission to ignore the
non-permission-requesting bits.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>