buffer: make touch_buffer() an exported function

We want to add a trace point to touch_buffer() but macros and inline
functions defined in header files can't have tracing points.  Move
touch_buffer() to fs/buffer.c and make it a proper function.

The new exported function is also declared inline.  As most uses of
touch_buffer() are inside buffer.c with nilfs2 as the only other user,
the effect of this change should be negligible.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>