xfs: allow SECURE namespace xattrs to use reserved block pool

We got a report from the podman folks that selinux relabels that happen
as part of their process were returning ENOSPC when the filesystem is
completely full. This is because xattr changes reserve about 15 blocks
for the worst case, but the common case is for selinux contexts to be
the sole, in-inode xattr and consume no blocks.

We already allow reserved space consumption for XFS_ATTR_ROOT for things
such as ACLs, and SECURE namespace attributes are not so very different,
so allow them to use the reserved space as well.

Code-comment-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

V2: Remove local variable, add comment.
V3: Add Dave's preferred comment
V4: Spelling and comment beautification