cxl: Fail mmap if requested mapping is larger than assigned problem state area

This patch makes the mmap call fail outright if the requested region is
larger than the problem state area assigned to the context so the error
is reported immediately rather than waiting for an attempt to access an
address out of bounds.

Although we never expect users to map more than the assigned problem
state area and are not aware of anyone doing this (other than for
testing), this does have the potential to break users if someone has
used a larger range regardless. I'm submitting it for consideration, but
if this change is not considered acceptable the previous patch is
sufficient to prevent access out of bounds without breaking anyone.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>