fuse: implement 'access' low level function

Add an empty 'access' function to fuse low level functions. This
allow us to use ceph-fuse with fuse_default_permissions = false.
'fuse_default_permissions = false' can significantly improve the
speed of create/removing large number of files.

When fuse_default_permissions is true, the fuse kernel module sends
a getattr request whenever the kernel needs to check a directory's
permission. getattr (STAT_CAP_INODE_ALL) can be very slow if the
directory was just modified.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1566 from ceph/wip-fuse-access

fuse: implement 'access' low level function

Reviewed-by: Sage Weil <sage@inktank.com>