LU-12652 ldiskfs: disable credits check in 4.18

LU-12652 ldiskfs: disable credits check in 4.18

4.18 has a credits check in xattr_set path with different math
resulting in false ENOSPC and subsequent failure to update LMA.
as a result - many tests fail on 4.18, especially with DNE.

Signed-off-by: Alexey Zhuravlev <bzzz@whamcloud.com>
Change-Id: I82b19c0c24988aed66d74cf2323e93acffb14f31
Reviewed-on: https://review.whamcloud.com/35750
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>