btrfs/file.c: remove an unsed varialbe first_index

The commit b37392ea ("Btrfs: cleanup unnecessary parameter
and variant of prepare_pages()") makes it redundant.
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Shan Hai <haishan.bai@hotmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -1469,7 +1469,6 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 	u64 release_bytes = 0;
 	u64 lockstart;
 	u64 lockend;
-	unsigned long first_index;
 	size_t num_written = 0;
 	int nrptrs;
 	int ret = 0;
@@ -1485,8 +1484,6 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 	if (!pages)
 		return -ENOMEM;
 
-	first_index = pos >> PAGE_CACHE_SHIFT;
-
 	while (iov_iter_count(i) > 0) {
 		size_t offset = pos & (PAGE_CACHE_SIZE - 1);
 		size_t write_bytes = min(iov_iter_count(i),