btrfs: Refactor btrfs_merge_bio_hook

This function really checks whether adding more data to the bio will
straddle a stripe/chunk. So first let's give it a more appropraite name
- btrfs_bio_fits_in_stripe. Secondly, the offset parameter was never
used to just remove it. Thirdly, pages are submitted to either btree or
data inodes so it's guaranteed that tree->ops is set so replace the
check with an ASSERT. Finally, document the parameters of the function.
No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -3191,9 +3191,8 @@ void btrfs_merge_delalloc_extent(struct inode *inode, struct extent_state *new,
 				 struct extent_state *other);
 void btrfs_split_delalloc_extent(struct inode *inode,
 				 struct extent_state *orig, u64 split);
-int btrfs_merge_bio_hook(struct page *page, unsigned long offset,
-			 size_t size, struct bio *bio,
-			 unsigned long bio_flags);
+int btrfs_bio_fits_in_stripe(struct page *page, size_t size, struct bio *bio,
+			     unsigned long bio_flags);
 void btrfs_set_range_writeback(struct extent_io_tree *tree, u64 start, u64 end);
 vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf);
 int btrfs_readpage(struct file *file, struct page *page);