ceph-fuse: add dedicated snap stag map for each directory

ceph-fuse: add dedicated snap stag map for each directory

This will fix the fino colliding bug, which is caused when the
snapid is later than 0xffff.

From mds 'mds_max_snaps_per_dir' option, we can see that the max
snapshots for each directory is 4_K, and in ceph-fuse we have
around 64_K, which is from 0xffff - 2, stags could be used to make
the fake fuse inode numbers for each directory.

Fixes: https://tracker.ceph.com/issues/54653
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit 7e4424a821f1f0162cc1f2bfddb1eb501e321702)