fs-verity: add SHA-512 support

Add SHA-512 support to fs-verity.  This is primarily a demonstration of
the trivial changes needed to support a new hash algorithm in fs-verity;
most users will still use SHA-256, due to the smaller space required to
store the hashes.  But some users may prefer SHA-512.

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

include/uapi/linux/fsverity.h

@@ -14,6 +14,7 @@
 #include <linux/types.h>
 
 #define FS_VERITY_HASH_ALG_SHA256	1
+#define FS_VERITY_HASH_ALG_SHA512	2
 
 struct fsverity_enable_arg {
 	__u32 version;