lfs_bd_cmp() compares more bytes at one time

It's very slowly to compare one byte at one time. Here are the
performance I get from 128M spinand with NFTL by sequential writing.

| file size | buffer size  | write speed  |
| 10 MB     | 0   B        | 3206.01 KB/s |
| 10 MB     | 1   B        | 2434.04 KB/s |
| 10 MB     | 2   B        | 2685.78 KB/s |
| 10 MB     | 4   B        | 2857.94 KB/s |
| 10 MB     | 8   B        | 3060.68 KB/s |
| 10 MB     | 16  B        | 3155.30 KB/s |
| 10 MB     | 64  B        | 3193.68 KB/s |
| 10 MB     | 128 B        | 3230.62 KB/s |
| 10 MB     | 256 B        | 3153.03 KB/s |

| 70 MB     | 0   B        | 2258.87 KB/s |
| 70 MB     | 1   B        | 1827.83 KB/s |
| 70 MB     | 2   B        | 1962.29 KB/s |
| 70 MB     | 4   B        | 2074.01 KB/s |
| 70 MB     | 8   B        | 2147.03 KB/s |
| 70 MB     | 64  B        | 2179.92 KB/s |
| 70 MB     | 256 B        | 2179.96 KB/s |

The 0 Byte size means no validation and the 1 Byte size is how
littlefs do before. Based on the above table and to save memory,
comparing 8 bytes at one time is more wonderful.

Signed-off-by: WeiXiong Liao <liaoweixiong@allwinnertech.com>

lfs.c

@@ -103,18 +103,22 @@ static int lfs_bd_cmp(lfs_t *lfs,
         lfs_block_t block, lfs_off_t off,
         const void *buffer, lfs_size_t size) {
     const uint8_t *data = buffer;
+    lfs_size_t diff = 0;
 
-    for (lfs_off_t i = 0; i < size; i++) {
-        uint8_t dat;
-        int err = lfs_bd_read(lfs,
+    for (lfs_off_t i = 0; i < size; i += diff) {
+        uint8_t dat[8];
+
+        diff = lfs_min(size-i, sizeof(dat));
+        int res = lfs_bd_read(lfs,
                 pcache, rcache, hint-i,
-                block, off+i, &dat, 1);
-        if (err) {
-            return err;
+                block, off+i, &dat, diff);
+        if (res) {
+            return res;
         }
 
-        if (dat != data[i]) {
-            return (dat < data[i]) ? LFS_CMP_LT : LFS_CMP_GT;
+        res = memcmp(dat, data + i, diff);
+        if (res) {
+            return res < 0 ? LFS_CMP_LT : LFS_CMP_GT;
         }
     }