selftests/mm: gup_longterm: new functional test for FOLL_LONGTERM

Let's add a new test for checking whether GUP long-term page pinning works
as expected (R/O vs.  R/W, MAP_PRIVATE vs.  MAP_SHARED, GUP vs. 
GUP-fast).  Note that COW handling with long-term R/O pinning in private
mappings, and pinning of anonymous memory in general, is tested by the COW
selftest.  This test, therefore, focuses on page pinning in file mappings.

The most interesting case is probably the "local tmpfile" case, as that
will likely end up on a "real" filesystem such as ext4 or xfs, not on a
virtual one like tmpfs or hugetlb where any long-term page pinning is
always expected to succeed.

For now, only add tests that use the "/sys/kernel/debug/gup_test"
interface.  We'll add tests based on liburing separately next.

[[email protected]: update .gitignore for gup_longterm, per Peter]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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