fs: 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.  Commits 801e5237
("fs: move generic stat response attr handling to vfs_getattr_nosec")
and 712b2698 ("fs/stat: Define DAX statx attribute") sets
STATX_ATTR_AUTOMOUNT and STATX_ATTR_DAX, respectively, without setting
stx_attributes_mask, which can cause xfstests generic/532 to fail.

Fix this in the same way as commit 1b9598c8 ("xfs: fix reporting
supported extra file attributes for statx()")

Fixes: 801e5237 ("fs: move generic stat response attr handling to vfs_getattr_nosec")
Fixes: 712b2698

 ("fs/stat: Define DAX statx attribute")
Cc: stable@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>