LU-17404 kernel: fix filemap_splice_read detection

LU-17404 kernel: fix filemap_splice_read detection

On Centos 9 kernel 5.14, filemap_splice_read is in the header files,
but the symbol is not exported by the kernel.
So instead of trying to build a kernel module with a call to this
function, just use LB_CHECK_EXPORT on this symbol.

Test-Parameters: trivial
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1f55d0b41c46a992204c1cebc3f5c8c7dbc6128e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55454
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>