btrfs: merge nr_pages input and output parameter in compress_pages

The parameter saying how many pages can be allocated at maximum can be
merged with the output page counter, to save some stack space.  The
compression implementation will sink the parameter to a local variable
so everything works as before.

The nr_pages variables can also be simply merged in compress_file_range
into one.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -914,8 +914,8 @@ static void free_workspaces(void)
  * Given an address space and start and length, compress the bytes into @pages
  * that are allocated on demand.
  *
- * @out_pages is used to return the number of pages allocated.  There
- * may be pages allocated even if we return an error.
+ * @out_pages is an in/out parameter, holds maximum number of pages to allocate
+ * and returns number of actually allocated pages
  *
  * @total_in is used to return the number of bytes actually read.  It
  * may be smaller than the input length if we had to exit early because we
@@ -930,7 +930,6 @@ static void free_workspaces(void)
  */
 int btrfs_compress_pages(int type, struct address_space *mapping,
 			 u64 start, struct page **pages,
-			 unsigned long nr_dest_pages,
 			 unsigned long *out_pages,
 			 unsigned long *total_in,
 			 unsigned long *total_out,
@@ -943,7 +942,7 @@ int btrfs_compress_pages(int type, struct address_space *mapping,
 
 	ret = btrfs_compress_op[type-1]->compress_pages(workspace, mapping,
 						      start, pages,
-						      nr_dest_pages, out_pages,
+						      out_pages,
 						      total_in, total_out,
 						      max_out);
 	free_workspace(type, workspace);