block: remove the bi_inline_vecs variable sized array from struct bio

block: remove the bi_inline_vecs variable sized array from struct bio

Bios are embedded into other structures, and at least spare is unhappy
about embedding structures with variable sized arrays.  There's no
real need to the array anyway, we can replace it with a helper pointing
to the memory just behind the bio, and with the previous cleanups there
is very few site (and only one outside the usual suspects) doing
anything special with it.

Signed-off-by: Christoph Hellwig <hch@lst.de>