selftests/bpf: Fix segmentation fault in test_progs

test_progs reports the segmentation fault as below:

  $ sudo ./test_progs -t mmap --verbose
  test_mmap:PASS:skel_open_and_load 0 nsec
  [...]
  test_mmap:PASS:adv_mmap1 0 nsec
  test_mmap:PASS:adv_mmap2 0 nsec
  test_mmap:PASS:adv_mmap3 0 nsec
  test_mmap:PASS:adv_mmap4 0 nsec
  Segmentation fault

This issue was triggered because mmap() and munmap() used inconsistent
length parameters; mmap() creates a new mapping of 3 * page_size, but the
length parameter set in the subsequent re-map and munmap() functions is
4 * page_size; this leads to the destruction of the process space.

To fix this issue, first create 4 pages of anonymous mapping, then do all
the mmap() with MAP_FIXED.

Another issue is that when unmap the second page fails, the length
parameter to delete tmp1 mappings should be 4 * page_size.

Signed-off-by: Jianlin Lv <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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