os/filestore: Fix erroneous WARNING: max attr too small

os/filestore: Fix erroneous WARNING: max attr too small

When we read the conf values from the conf file we call handle_conf_change which
calls set_xattr_limits_via_conf but this is before m_fs_type is initialised so
we fall through to the default comaprison with
filestore_max_xattr_value_size_other (1024) and erroneously issue the warning.
This happens at every startup but subsequent calls once the filestore is mounted
proceed as expected. Don't call set_xattr_limits_via_conf unless the filestore
is mounted.

Fixes: http://tracker.ceph.com/issues/17420
Signed-off-by: Brad Hubbard <bhubbard@redhat.com>