mds: rollback the snapdiff fragment entries with the same name if needed.

mds: rollback the snapdiff fragment entries with the same name if needed.

This is required when more entries with the same name don't fit into the
fragment. With the existing means for fragment offset specification such a splitting to be
prohibited.

Resolves: rhbz#2390060

Fixes: https://tracker.ceph.com/issues/72518
Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>
(cherry picked from commit 24955e66f4826f8623d2bec1dbfc580f0e4c39ae)
(cherry picked from commit 0a9e33733c50aacac223bd409813b0a711b7b181)