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>
Reviewed-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Closes: https://gitlab.freedesktop.org/wayland/wayland/-/issues/235