strbuf: Add strbuf_reserve_extra()

strbuf: Add strbuf_reserve_extra()

To accomplish the same task as scratchbuf_alloc() does: make sure the
buffer has enough space for next operations. One difference is that
ensures **extra** space, not the total space. If needed in future,
a strbuf_reserve(), that resembles C++'s std::vector::reserve(), can be
added on top.

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/239