LU-12634 llite: Use __xa_set_mark if it is available

LU-12634 llite: Use __xa_set_mark if it is available

Linux v4.19-rc5-248-g9b89a0355144
xarray: Add XArray marks

Test for and use __xa_set_mark() for marking page cache pages.

Move kernel compat wrappers in to inline functions.
Co-locate the configure test macros for 4.20 in kernel version order.

Test-Parameters: trivial
Cray-bug-id: LUS-7689
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I659a0e3af0a648d50205f44f2649ba8b982bfa42
Reviewed-on: https://review.whamcloud.com/36373
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>