nfsd: wrap accesses to st_access_bmap

Currently, we do this for the most part with "bare" bitops, but
eventually we'll need to expand the share mode code to handle access
and deny modes on other nodes.

In order to facilitate that code in the future, move to some generic
accessor functions. For now, these are mostly static inlines, but
eventually we'll want to move these to "real" functions that are
able to handle multi-node configurations or have a way to "swap in"
new operations to be done in lieu of or in conjunction with these
atomic bitops.

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