gallium/rtasm: handle mmap failures appropriately

gallium/rtasm: handle mmap failures appropriately

For a variety of reasons mmap (selinux and pax to name
a few) and can fail and with current code. This will
result in a crash in the driver, if not worse.

This has been the case since the inception of the
gallium copy of rtasm.

Cc: 9.1 9.2 10.0 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73473
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>