generic/611: Use _getfattr instead of GETFATTR_PROG

generic/611: Use _getfattr instead of GETFATTR_PROG

When using old version(such as getfattr 2.4.46) getfattr command, it
has the following output,

touch file
setfattr -n user.a file
getfattr --absolute-names -n user.a file
user.a

on new getfattr, it reports the following output,
getfattr --absolute-names -n user.a file
user.a=""

The {=""} will break the golden image, so use _getfattr to filter
the redundant ="" at the end if it has.

Signed-off-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>