x86/boot: Fix memory leak in default_get_smp_config()

When default_get_smp_config() is called with early == 1 and mpf->feature1
is non-zero, mpf is leaked because the return path does not do
early_memunmap().

Fix this and share a common exit routine.

Fixes: 5997efb9 ("x86/boot: Use memremap() to map the MPF and MPC data")
Reported-by: Cfir Cohen <cfir@google.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1907091942570.28240@chino.kir.corp.google.com