Add Linus' optimized sha1 hash for checksum

It does appear to be pretty fast.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/verify.h b/verify.h
index 50c8e43..c12bc7d 100644
--- a/verify.h
+++ b/verify.h
@@ -15,6 +15,7 @@
 	VERIFY_SHA256,			/* sha256 sum data blocks */
 	VERIFY_SHA512,			/* sha512 sum data blocks */
 	VERIFY_META,			/* block_num, timestamp etc. */
+	VERIFY_SHA1,			/* sha1 sum data blocks */
 	VERIFY_NULL,			/* pretend to verify */
 };
 
@@ -38,6 +39,9 @@
 struct vhdr_sha256 {
 	uint8_t sha256[128];
 };
+struct vhdr_sha1 {
+	uint32_t sha1[5];
+};
 struct vhdr_crc64 {
 	uint64_t crc64;
 };