selftests/mm: Update va_high_addr_switch.sh to check CPU for la57 flag

In order for the page table level 5 to be in use, the CPU must have the
setting enabled in addition to the CONFIG option. Check for the flag to be
set to avoid false test failures on systems that do not have this cpu flag
set.

The test does a series of mmap calls including three using the
MAP_FIXED flag and specifying an address that is 1<<47 or 1<<48.  These
addresses are only available if you are using level 5 page tables,
which requires both the CPU to have the capabiltiy (la57 flag) and the
kernel to be configured.  Currently the test only checks for the kernel
configuration option, so this test can still report a false positive. 
Here are the three failing lines:

$ ./va_high_addr_switch | grep FAILED
mmap(ADDR_SWITCH_HINT, 2 * PAGE_SIZE, MAP_FIXED): 0xffffffffffffffff - FAILED
mmap(HIGH_ADDR, MAP_FIXED): 0xffffffffffffffff - FAILED
mmap(ADDR_SWITCH_HINT, 2 * PAGE_SIZE, MAP_FIXED): 0xffffffffffffffff - FAILED

I thought (for about a second) refactoring the test so that these three
mmap calls will only be run on systems with the level 5 page tables
available, but the whole point of the test is to check the level 5
feature...

Link: https://lkml.kernel.org/r/20240119205801.62769-1-audra@redhat.com
Fixes: 4f2930c6

 ("selftests/vm: only run 128TBswitch with 5-level paging")
Signed-off-by: Audra Mitchell <audra@redhat.com>
Cc: Rafael Aquini <raquini@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Adam Sindelar <adam@wowsignal.io>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>