fs: uapi: define verity bit for FS_IOC_GETFLAGS

Add FS_VERITY_FL to the flags for FS_IOC_GETFLAGS, so that applications
can easily determine whether a file is a verity file at the same time as
they're checking other file flags.  This flag will be gettable only;
FS_IOC_SETFLAGS won't allow setting it, since an ioctl must be used
instead to provide more parameters.

This flag matches the on-disk bit that was already allocated for ext4.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>