shm: Close file descriptors not needed

Commit 5a981ee8 implemented a fallback path for platforms which do not
support mremap() such as FreeBSD.

To do so, the file descriptor for the mmap() is not closed immediately
but instead kept as long as the pool exists.

That induces more file descriptors kept open for longer, which in turn
may cause problems as wl_shm may be using a lot of file descriptors,
especially with Xwayland which can create a lot of pixmaps on behalf of
its X11 clients.

For platforms where mremap() is available, keeping those file
descriptors opened is a bit of a waste and may cause exhaustion of file
descriptors sooner that before commit 5a981ee8.

Only keep the mmap() file descriptor open on platforms which do not
implement mremap()  and close it immediately as before on others.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Closes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1283

src/wayland-shm.c

@@ -65,10 +65,12 @@ struct wl_shm_pool {
 	char *data;
 	ssize_t size;
 	ssize_t new_size;
+#ifndef MREMAP_MAYMOVE
 	/* The following three fields are needed for mremap() emulation. */
 	int mmap_fd;
 	int mmap_flags;
 	int mmap_prot;
+#endif
 	bool sigbus_is_impossible;
 };
 
@@ -153,7 +155,9 @@ shm_pool_unref(struct wl_shm_pool *pool, bool external)
 		return;
 
 	munmap(pool->data, pool->size);
+#ifndef MREMAP_MAYMOVE
 	close(pool->mmap_fd);
+#endif
 	free(pool);
 }
 
@@ -344,10 +348,14 @@ shm_create_pool(struct wl_client *client, struct wl_resource *resource,
 				       strerror(errno));
 		goto err_free;
 	}
+#ifndef MREMAP_MAYMOVE
 	/* We may need to keep the fd, prot and flags to emulate mremap(). */
 	pool->mmap_fd = fd;
 	pool->mmap_prot = prot;
 	pool->mmap_flags = flags;
+#else
+	close(fd);
+#endif
 	pool->resource =
 		wl_resource_create(client, &wl_shm_pool_interface, 1, id);
 	if (!pool->resource) {