wayland-shm: Don’t set SIGBUS handlers on unshrinkable fd

If a client set the F_SEAL_SHRINK seal on the fd before passing it to
the compositor, the kernel will ensure this fd won’t be able to shrink,
ever.  This allows us to avoid setting up the SIGBUS handlers on such
file descriptors.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>