LU-11974 llapi: improve llapi_layout_get_by_xattr(3) API

LU-11974 llapi: improve llapi_layout_get_by_xattr(3) API

llapi_layout_get_by_xattr() assumes that the lum has already
been properly swapped by llapi_layout_swab_lov_user_md().
However, llapi_layout_swab_lov_user_md() function is not
exported, so external tool won't be able to use it.

Instead of exporting a lot of APIs, this patch include the
swab functions into llapi_layout_get_by_xattr() and add an
argument flags to the API.

Change-Id: I9fbf0f0ba66660d2f382fb20b03f069c1a7afad5
Signed-off-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/34276
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>