cgfs: make dorealloc allocate the first batch, too

cgfs: make dorealloc allocate the first batch, too

With a short first line the case can be
 *mem = NULL
 oldlen = 0
 newlen = 5 (anything < 50)
making newbatches == oldbatches == 1 causing the
 (newbatches <= oldbatches)
condition to be true.

Let realloc() handle *mem==NULL and use
(!*mem || newbatches > oldbatches) as the only condition.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>