xfs: fix reporting supported extra file attributes for statx()

xfs: fix reporting supported extra file attributes for statx()

statx(2) notes that any attribute that is not indicated as supported by
stx_attributes_mask has no usable value. Commit 856b9dd98372e1 ("xfs: report
crtime and attribute flags to statx") added support for informing userspace
of extra file attributes but forgot to list these flags as supported
making reporting them rather useless for the pedantic userspace author.

$ git describe --contains 856b9dd98372e129486c3b8256e14661159b4065
v4.11-rc6~5^2^2~2

Fixes: 856b9dd98372e1 ("xfs: report crtime and attribute flags to statx")
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: add a comment reminding people to keep attributes_mask up to date]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>