sh: Plug PMB alloc memory leak

If we fail to allocate a PMB entry in pmb_remap() we must remember to
clear and free any PMB entries that we may have previously allocated,
e.g. if we were allocating a multiple entry mapping.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>