LU-9183 ptlrpc: handle posix_acl: xattr representation cleanups

LU-9183 ptlrpc: handle posix_acl: xattr representation cleanups

In commit 2211d5ba5c6c4e972ba6dbc912b2897425ea6621 typedef of
posix_acl_xattr_{header,entry} was removed and replaced with
struct posix_acl_xattr_{header,entry} declaration. Also member
a_entries was removed from struct posix_acl_xattr_header.

Since we have to support older kernels this patch does the
opposite of what was done upstream in that we treat struct
posix_acl_xattr_{header,entry} of newer kernels as the typedef
used by older kernels.

Change-Id: I880f73d495dc6c35a1671b49e509350118f6e154
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: https://review.whamcloud.com/25892
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>