xattr: Option to disable meta-data block cache

mbcache provides absolutely no value for Lustre xattrs (because
they are unique and cannot be shared between files) and as we can
see it has a noticable overhead in some cases. In the past there
was a CONFIG_MBCACHE option that would allow it to be disabled,
but this was removed in newer kernels, so we will need to patch
ldiskfs to fix this.

References: <https://bugzilla.kernel.org/show_bug.cgi?id=107301>
References: <https://git.hpdd.intel.com/fs/lustre-release.git/blob_plain/HEAD:/ldiskfs/kernel_patches/patches/rhel7/ext4-disable-mb-cache.patch

>
References: CVE-2015-8952

On 13.12.2016 at 15:58 Ben Hutchings wrote:
> I decided not to apply this as it's a userland ABI extension that we
> would then need to carry indefinitely.

Signed-off-by: Philipp Hahn <hahn@univention.de>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>