LU-13430 vfs: add ll_vfs_getxattr/ll_vfs_setxattr compat macro

LU-13430 vfs: add ll_vfs_getxattr/ll_vfs_setxattr compat macro

Instead of checking everywhere in the code whether kernel defines
__vfs_getxattr/__vfs_setxattr/__vfs_removexattr, define compat macros
to handle this in lustre_compat.h.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iabb54c41e6ccc4b9524f614308a4cd98c5a7ec55
Reviewed-on: https://review.whamcloud.com/38175
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>