statx: define STATX_ATTR_VERITY

Add a statx attribute bit STATX_ATTR_VERITY which will be set if the
file has fs-verity enabled.  This is the statx() equivalent of
FS_VERITY_FL which is returned by FS_IOC_GETFLAGS.

This is useful because it allows applications to check whether a file is
a verity file without opening it.  Opening a verity file can be
expensive because the fsverity_info is set up on open, which involves
parsing metadata and optionally verifying a cryptographic signature.

This is analogous to how various other bits are exposed through both
FS_IOC_GETFLAGS and statx(), e.g. the encrypt bit.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Acked-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>