selftest/vm: fix map_fixed_noreplace test failure

selftest/vm: fix map_fixed_noreplace test failure

On the latest RHEL the test fails due to executable mapped at 256MB
address

     # ./map_fixed_noreplace
    mmap() @ 0x10000000-0x10050000 p=0xffffffffffffffff result=File exists
    10000000-10010000 r-xp 00000000 fd:04 34905657                           /root/rpmbuild/BUILD/kernel-5.14.0-56.el9/linux-5.14.0-56.el9.ppc64le/tools/testing/selftests/vm/map_fixed_noreplace
    10010000-10020000 r--p 00000000 fd:04 34905657                           /root/rpmbuild/BUILD/kernel-5.14.0-56.el9/linux-5.14.0-56.el9.ppc64le/tools/testing/selftests/vm/map_fixed_noreplace
    10020000-10030000 rw-p 00010000 fd:04 34905657                           /root/rpmbuild/BUILD/kernel-5.14.0-56.el9/linux-5.14.0-56.el9.ppc64le/tools/testing/selftests/vm/map_fixed_noreplace
    10029b90000-10029bc0000 rw-p 00000000 00:00 0                            [heap]
    7fffbb510000-7fffbb750000 r-xp 00000000 fd:04 24534                      /usr/lib64/libc.so.6
    7fffbb750000-7fffbb760000 r--p 00230000 fd:04 24534                      /usr/lib64/libc.so.6
    7fffbb760000-7fffbb770000 rw-p 00240000 fd:04 24534                      /usr/lib64/libc.so.6
    7fffbb780000-7fffbb7a0000 r--p 00000000 00:00 0                          [vvar]
    7fffbb7a0000-7fffbb7b0000 r-xp 00000000 00:00 0                          [vdso]
    7fffbb7b0000-7fffbb800000 r-xp 00000000 fd:04 24514                      /usr/lib64/ld64.so.2
    7fffbb800000-7fffbb810000 r--p 00040000 fd:04 24514                      /usr/lib64/ld64.so.2
    7fffbb810000-7fffbb820000 rw-p 00050000 fd:04 24514                      /usr/lib64/ld64.so.2
    7fffd93f0000-7fffd9420000 rw-p 00000000 00:00 0                          [stack]
    Error: couldn't map the space we need for the test

Fix this by finding a free address using mmap instead of hardcoding
BASE_ADDRESS.

Link: https://lkml.kernel.org/r/20220217083417.373823-1-aneesh.kumar@linux.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Jann Horn <jannh@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>