LU-18813 osd: fix dcb_func LASSERT

LU-18813 osd: fix dcb_func LASSERT

Each OSD was incorrectly asserting that the
address of the function pointer was not NULL,
instead of the function pointer itself.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ie5682a9d80219743ecb86d8d463cbabcdbf77b64
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59304
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>