libgralloc : Correct the usage of pmem ioctl

Return error only if the ioctl PMEM_ALLOCATE_ALIGNED
returns negative value.

CRs-Fixed: 375513

(cherry picked from commit b1caf041824cedc268ced33e47a166070ec84f29)

Change-Id: Ie5e926648ff845dc04ce06e5c57e27dd214d1b45