mds: don't shallow copy when decoding xattr map

mds: don't shallow copy when decoding xattr map

Otherwise inodes' xattr maps may reference a large shared buffer
(from omap fetch or journal read). If mds trims and loads inode
repeatly, each inode can reference different large buffer in the
worst case.

Fixes: https://tracker.ceph.com/issues/45090
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
(cherry picked from commit deff94c8f43bb3734b688ccb828d942b8f150638)

Conflicts:
	src/mds/CInode.cc
- CInode::decode_lock_ixattr is missing in nautilus
	src/mds/mdstypes.h
- nautilus has a code comment that is not in master