LU-17052 libcfs: fix build for old kernel

LU-17052 libcfs: fix build for old kernel

Fix build for kernel v4.17 to v4.19.
These old kernels already have xarray.h and #include by fs.h but
don't have full xarray support. It is needed to #include libcfs's
xarray.h also to contain xarray support.

Rename the header define macro to ensure libcfs's xarray.h will be
included。

Test-Parameters: trivial
Test-Parameters: testlist=sanityn envdefinitions=ONLY=77,ONLY_REPEAT=20
Fixes: 778791dd7da1 ("LU-8130 libcfs: don't use radix tree for xarray")
Change-Id: I760c394cc1d885c2de79d1770243ab7f292b9b3a
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52090
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>