btrfs: remove set but not used variable 'num_pages'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/btrfs/ioctl.c: In function 'btrfs_extent_same':
fs/btrfs/ioctl.c:3260:6: warning:
 variable 'num_pages' set but not used [-Wunused-but-set-variable]

It not used any more since commit 9ee8234e6220 ("Btrfs: use
generic_remap_file_range_prep() for cloning and deduplication")
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3278,7 +3278,6 @@ static int btrfs_extent_same(struct inode *src, u64 loff, u64 olen,
 			     struct inode *dst, u64 dst_loff)
 {
 	int ret;
-	int num_pages = PAGE_ALIGN(BTRFS_MAX_DEDUPE_LEN) >> PAGE_SHIFT;
 	u64 i, tail_len, chunk_count;
 
 	/* don't make the dst file partly checksummed */
@@ -3291,8 +3290,6 @@ static int btrfs_extent_same(struct inode *src, u64 loff, u64 olen,
 
 	tail_len = olen % BTRFS_MAX_DEDUPE_LEN;
 	chunk_count = div_u64(olen, BTRFS_MAX_DEDUPE_LEN);
-	if (chunk_count == 0)
-		num_pages = PAGE_ALIGN(tail_len) >> PAGE_SHIFT;
 
 	for (i = 0; i < chunk_count; i++) {
 		ret = btrfs_extent_same_range(src, loff, BTRFS_MAX_DEDUPE_LEN,