mds: just wait the client flushes the snap and dirty buffer

mds: just wait the client flushes the snap and dirty buffer

When truncating the inode we will just set the ifile lock state to
LOCK_XLOCKSNAP and then try to revoke the 'Fb' caps, but if the
client couldn't release the 'Fb' cap in time just replies with a
normal cap updating request, the MDS will successfully transfer
the ifile's lock state to LOCK_EXCL, which is stable.

That means the MDS will wake up the truncating request and continue
truncating the objects from Rados without waiting the clients to
flush the diry buffer.

Fixes: commit 9c65920e7f6 ("mds: force client flush snap data before
		truncating objects")
Fixes: https://tracker.ceph.com/issues/62580
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit 68179ae21384b70c284026ac2b3dbf9f318e9af7)