Add S_VERITY and IS_VERITY()

Similar to S_ENCRYPTED/IS_ENCRYPTED(), this commit adds
S_VERITY/IS_VERITY() to be able to check if a VFS inode has verity
information associated with it.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>