mmu: fix iovecs initialization in fill_file_page

Value passed to std::vector constructor increase not only vector
capacity() but also its size(). We want only former, not later and it
is achieved by reserve() function.

Also remove prev_off which is a leftover of a debug code.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>