rbd: passing wrong variable to bvec_kunmap_irq()

We should be passing "buf" here insead of "bv".  This is tricky because
it's not the same as kmap() and kunmap().  GCC does warn about it if you
compile on i386 with CONFIG_HIGHMEM.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>