dm-verity: make dm_verity_fec_io::bufs variable-length

dm-verity: make dm_verity_fec_io::bufs variable-length

When correcting a data block, the FEC code performs optimally when it
has enough buffers to hold all the needed RS blocks.  That number of
buffers is '1 << (v->data_dev_block_bits - DM_VERITY_FEC_BUF_RS_BITS)'.

However, since v->data_dev_block_bits isn't a compile-time constant, the
code actually used PAGE_SHIFT instead.

With the traditional PAGE_SIZE == data_block_size == 4096, this was
fine.  However, when PAGE_SIZE > data_block_size, this wastes space.
E.g., with data_block_size == 4096 && PAGE_SIZE == 16384, struct
dm_verity_fec_io is 9240 bytes, when in fact only 3096 bytes are needed.

Fix this by making dm_verity_fec_io::bufs a variable-length array.

This makes the macros DM_VERITY_FEC_BUF_MAX and
fec_for_each_extra_buffer() no longer apply, so remove them.  For
consistency, and because DM_VERITY_FEC_BUF_PREALLOC is fixed at 1 and
was already assumed to be 1 (considering that mempool_alloc() shouldn't
be called in a loop), also remove the related macros
DM_VERITY_FEC_BUF_PREALLOC and fec_for_each_prealloc_buffer().

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>