nfsd: ensure that st_access_bmap and st_deny_bmap are initialized to 0

Open stateids must be initialized with the st_access_bmap and
st_deny_bmap set to 0, so that nfs4_get_vfs_file can properly record
their state in old_access_bmap and old_deny_bmap.

This bug was introduced in commit baeb4ff0 (nfsd: make deny mode
enforcement more efficient and close races in it) and was causing the
refcounts to end up incorrect when nfs4_get_vfs_file returned an error
after bumping the refcounts. This made it impossible to unmount the
underlying filesystem after running pynfs tests that involve deny modes.

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