LU-14073 build: fix autoconf test for clean_bdev_aliases()

LU-14073 build: fix autoconf test for clean_bdev_aliases()

From 5.9, buffer_head.h no longer provides a declaration for
'struct block_device' so the code fragment fails because the compiler
doesn't know the size of that structure.

Instead, simple pass NULL rather than the address of a real structure.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I1775572fbd56d22822b6e440fe95bd105042e7b8
Reviewed-on: https://review.whamcloud.com/40395
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>