This patch adds support for the max_files quota.  It hooks into all the
ceph functions that add new filesystem objects that need to be checked
against the quota limits.  When these limits are hit, -EDQUOT is returned.

Note that we're not checking quotas on ceph_link().  ceph_link doesn't
really create a new inode,  and since the MDS doesn't update the directory
statistics when a new (hard) link is created (only with symlinks), they
are not accounted as a new file.
Signed-off-by: NLuis Henriques <lhenriques@suse.com>
Reviewed-by: N"Yan, Zheng" <zyan@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>