These are more efficient than separate pin/unpin calls because
pin count increment and decrement can be done cheaply when the
data pointer is known.
Secondly, pinned access can be made safe against preemption by
hardware interrupts or other CPU cores; buflib_get_data() can't.
This makes it more useful under different threading models and
for SMP targets; both of which are not particularly relevant to
Rockbox now, but might be in the future.
Change-Id: I09284251b83bbbc59ef88a494c8fda26a7f7ef26
