Add engine flag FIO_SKIPPABLE_IOMEM_ALLOC

Add engine flag FIO_SKIPPABLE_IOMEM_ALLOC

It makes it valid to set option mem/iomem even when the engine specifies
iomem_alloc and iomem_free callbacks, allowing users to optionally use
fio's customizable memory allocation logic instead of the engine's.

This is in preparation for giving libblkio engine users the choice
between controlling memory allocation or delegating it to the libblkio
library.

Signed-off-by: Alberto Faria <afaria@redhat.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>