btrfs/254: correct subject of the relevant kernel patch and add git commit

btrfs/254: correct subject of the relevant kernel patch and add git commit

The top comment of test btrfs/254 mentions a kernel patch with a subject
of:

   "btrfs: harden identification of the stale device"

but that is actually not correct, as the subject was slightly changed when
the patch was picked to:

   "btrfs: harden identification of a stale device"

So fix that by removing the comment and use instead a call to
_fixed_by_kernel_commit, which also allows us to specify git commit id.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>