selftests: mm: fix build errors on armhf

selftests: mm: fix build errors on armhf

The __NR_mmap isn't found on armhf.  The mmap() is commonly available
system call and its wrapper is present on all architectures.  So it should
be used directly.  It solves problem for armhf and doesn't create problem
for other architectures.

Remove sys_mmap() functions as they aren't doing anything else other than
calling mmap().  There is no need to set errno = 0 manually as glibc
always resets it.

For reference errors are as following:

  CC       seal_elf
seal_elf.c: In function 'sys_mmap':
seal_elf.c:39:33: error: '__NR_mmap' undeclared (first use in this function)
   39 |         sret = (void *) syscall(__NR_mmap, addr, len, prot,
      |                                 ^~~~~~~~~

mseal_test.c: In function 'sys_mmap':
mseal_test.c:90:33: error: '__NR_mmap' undeclared (first use in this function)
   90 |         sret = (void *) syscall(__NR_mmap, addr, len, prot,
      |                                 ^~~~~~~~~

Link: https://lkml.kernel.org/r/20240809082511.497266-1-usama.anjum@collabora.com
Fixes: 4926c7a52de7 ("selftest mm/mseal memory sealing")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>