mds: make the lost+found dir accessible from clients

mds: make the lost+found dir accessible from clients

Inode number 0x4 is reserved for the lost+found dir, and the apps
or recovery apps need to access it.

At the same time the 0x3 is reserved for the global snaprealm, which
will also be lookup by the clients when looking up the snaprealm inodes.

Here will make all the inode less than 100 accessible by clients.

Fixes: https://tracker.ceph.com/issues/50216
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit 841071b3602aa2b8bcca57b95cdf671d48036a34)

Conflicts:
	src/mds/Server.cc
- most of the master commit was already backported via
  c5362b8464bdafbea7556acdee9e877b71ed4f8d
  This backports just one small part that was missed in that commit.