LU-11417 llapi: add llapi_layout_get_by_xattr(3) API

LU-11417 llapi: add llapi_layout_get_by_xattr(3) API

Add new llapi_layout_get_by_xattr(3) interface to be able to extract
a layout structure from a LOV EA xattr.  This can be useful when the
xattr is retrieved from some external source (e.g. tarball, HSM, or
tools that directly access the underlying ldiskfs or ZFS filesystem).

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I9b405bf6b3119e44097f36d49ac5859ff93ebbe5
Reviewed-on: https://review.whamcloud.com/33230
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>