[SCSI] st: rename flush_write_buffer()

This patch fixes the following namespace collision with
include/asm-avr32/cacheflush.h :

<--  snip  -->

...
  CC [M]  drivers/scsi/st.o
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/scsi/st.c:629:53: error: macro "flush_write_buffer" passed 1 arguments, but takes just 0
...
make[3]: *** [drivers/scsi/st.o] Error 1

<--  snip  -->

st now uses st_flush_write_buffer()

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>