selftests/vm: retry on EAGAIN for MADV_COLLAPSE selftest

MADV_COLLAPSE is a best-effort request that will set errno to an
actionable value if the request cannot be performed.

For example, if pages are not found on the LRU, or if they are currently
locked by something else, MADV_COLLAPSE will fail and set errno to EAGAIN
to inform callers that they may try again.

Since the khugepaged selftest is the first public use of MADV_COLLAPSE,
set a best practice of checking errno and retrying on EAGAIN.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9330694de59f ("selftests/vm: add MADV_COLLAPSE collapse context to selftests")
Signed-off-by: Zach O'Keefe <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Chris Kennelly <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: James Houghton <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Rongwei Wang <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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