mds: always store backtrace xattr in the default pool

when creating file in non-default pool, we need to store a backtrace
in the default pool in addition to the specified pool. Otherwise the
'lookup-by-ino' function will consider backtrace for file is missing.

Fixes: #6914
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: don't allow changing layout for non-regular file

layout is meaningless for non-regular file. using non-default layout
may make backtrace less efficiency.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1029 from ceph/wip-6914

Wip 6914

Reviewed-by: Sage Weil <sage@inktank.com>