mon: add new configuration to limit filesystem xattrs size

mon: add new configuration to limit filesystem xattrs size

This new configuration option will allow to define the maximum size for a
filesystem xattrs blob.  This is a filesystem-wide knob that will replace
the per-MDS mds_max_xattr_pairs_size option.

Note:

The kernel client patch to handle this new configuration was merged before
the corresponding ceph-side pull-request.  This was unfortunate because in
the meantime PR #43284 was merged and the encoding/decoding of
'bal_rank_mask' got in between.  Hence the 'max_xattr_size' is being
encoding/decoded before 'bal_rank_mask'.

URL: https://tracker.ceph.com/issues/55725
Signed-off-by: Luís Henriques <lhenriques@suse.de>
(cherry picked from commit 7b8def5c30b92ec642209f6ab1af312e49343c01)