cxl: Fix off by one error allowing subsequent mmap page to be accessed

It was discovered that if a process mmaped their problem state area they
were able to access one page more than expected, potentially allowing
them to access the problem state area of an unrelated process.

This was due to a simple off by one error in the mmap fault handler
introduced in 0712dc7e ("cxl: Fix issues
when unmapping contexts"), which is fixed in this patch.

Cc: stable@vger.kernel.org
Fixes: 0712dc7e ("cxl: Fix issues when unmapping contexts")
Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>