wayland: fix error handling for memfd_create

wayland: fix error handling for memfd_create

We currently use syscall() directly to invoke memfd_create,
since the function isn't available in libc headers yet.

The code, though, mishandles how errors are passed from syscall().
It assumes syscall returns the error code directly (but negative),
when in fact, syscall() uses errno.

Also, the code fails to retry on EINTR.

This commit moves the handling of memfd create to a helper function,
and changes the code to use errno and handle EINTR.

https://bugzilla.gnome.org/show_bug.cgi?id=766341