nfsd: split up nfsd_setattr

commit 818e5a22e907fbae75e9c1fd78233baec9fa64b6 upstream.

Split out two helpers to make the code more readable and easier to verify
for correctness.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
[bwh: Backported to 3.2: s/umode_t/int/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>