nfsd: consolidate set_access and set_deny

These functions are identical. Also, rename them to bmap_to_share_mode
to better reflect what they do, and have them just return the result
instead of passing in a pointer to the storage location.

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