selftests/vm: fix map_hugetlb length used for testing read and write

selftests/vm: fix map_hugetlb length used for testing read and write

Commit fa7b9a805c79 ("tools/selftest/vm: allow choosing mem size and page
size in map_hugetlb") added the possibility to change the size of memory
mapped for the test, but left the read and write test using the default
value.  This is unnoticed when mapping a length greater than the default
one, but segfaults otherwise.

Fix read_bytes() and write_bytes() by giving them the real length.

Also fix the call to munmap().

Fixes: fa7b9a805c79 ("tools/selftest/vm: allow choosing mem size and page size in map_hugetlb")
Signed-off-by: Christophe Leroy <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Leonardo Bras <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: <[email protected]>
Link: http://lkml.kernel.org/r/9a404a13c871c4bd0ba9ede68f69a1225180dd7e.1580978385.git.christophe.leroy@c-s.fr
Signed-off-by: Linus Torvalds <[email protected]>