LU-13839 ldiskfs: fix detecting ext4_journal_ensure_credits

LU-13839 ldiskfs: fix detecting ext4_journal_ensure_credits

As __ext4_journal_ensure_credits() is not exported, the current
detection doesn't work.
So add a local definition so that the build succeeds.

Fixes: cf48a2d23bca ("LU-13839 kernel: RHEL 8.3 server support")
Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I5f4b0883e8425d26760ea77e89379103dce30855
Reviewed-on: https://review.whamcloud.com/40939
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>