shm: Relax shm_pool_create_buffer() validity check

shm_pool_create_buffer() can raise a false WL_SHM_ERROR_INVALID_STRIDE error under some circumstances because of integer division.

Fix this by using a strict comparison operator instead of lower or equal.

Signed-off-by: Olivier Fourdan ofourdan@redhat.com Closes: #235 (closed)