mds: Fix readdir when osd is full.

mds: Fix readdir when osd is full.

Problem:
The readdir wouldn't list all the entries in the directory
when the osd is full with rstats enabled.

Cause:
The issue happens only in multi-mds cephfs cluster. If rstats
is enabled, the readdir would request 'Fa' cap on every dentry,
basically to fetch the size of the directories. Note that 'Fa' is
CEPH_CAP_GWREXTEND which maps to CEPH_CAP_FILE_WREXTEND and is
used by CEPH_STAT_RSTAT.

The request for the cap is a getattr call and it need not go to
the auth mds. If rstats is enabled, the getattr would go with
the mask CEPH_STAT_RSTAT which mandates the requirement for
auth-mds in 'handle_client_getattr', so that the request gets
forwarded to auth mds if it's not the auth. But if the osd is full,
the indode is fetched in the 'dispatch_client_request' before
calling the  handler function of respective op, to check the
FULL cap access for certain metadata write operations. If the inode
doesn't exist, ESTALE is returned. This is wrong for the operations
like getattr, where the inode might not be in memory on the non-auth
mds and returning ESTALE is confusing and client wouldn't retry. This
is introduced by the commit 6db81d8479b539d which fixes subvolume
deletion when osd is full.

Fix:
Fetch the inode required for the FULL cap access check for the
relevant operations in osd full scenario. This makes sense because
all the operations would mostly be preceded with lookup and load
the inode in memory or they would handle ESTALE gracefully.

Fixes: https://tracker.ceph.com/issues/72260
Introduced-by: 6db81d8479b539d3ca6b98dc244c525e71a36437
Signed-off-by: Kotresh HR <khiremat@redhat.com>
(cherry picked from commit 1ca8f334f944ff78ba12894f385ffb8c1932901c)