Add option for specifically setting buffer contents

Fio can use zeroes, slightly scrambled data, full random data,
or specifically compressible data. With this option, the user
can now fully control the pattern written, similarly to how
verify_pattern works for verify=meta.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/verify.h b/verify.h
index 6a81e9b..05d7b81 100644
--- a/verify.h
+++ b/verify.h
@@ -74,7 +74,8 @@
 extern int __must_check get_next_verify(struct thread_data *td, struct io_u *);
 extern int __must_check verify_io_u(struct thread_data *, struct io_u *);
 extern int verify_io_u_async(struct thread_data *, struct io_u *);
-extern void fill_pattern(struct thread_data *td, void *p, unsigned int len, struct io_u *io_u, unsigned long seed, int use_seed);
+extern void fill_verify_pattern(struct thread_data *td, void *p, unsigned int len, struct io_u *io_u, unsigned long seed, int use_seed);
+extern void fill_buffer_pattern(struct thread_data *td, void *p, unsigned int len);
 extern void fio_verify_init(struct thread_data *td);
 
 /*