Added const to fill_buffer

src/core/buffer.c

  *
  * Returns 1 for success, 0 for failure (e.g. buffer too small).
  */
-int fill_buffer ( struct buffer *buffer, void *data,
+int fill_buffer ( struct buffer *buffer, const void *data,
 		  off_t offset, size_t len ) {
 	struct buffer_free_block desc;
 	physaddr_t block, prev_block;

src/include/buffer.h

 /* Functions in buffer.c */
 
 extern void init_buffer ( struct buffer *buffer );
-extern int fill_buffer ( struct buffer *buffer, void *data,
+extern int fill_buffer ( struct buffer *buffer, const void *data,
 			 off_t offset, size_t len );
 
 #endif /* BUFFER_H */