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shmem_populate: avoid an useless check, and some comments
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From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

Either shmem_getpage returns a failure, or it found a page, or it was told it
couldn't do any I/O. So it's useless to check nonblock in the else branch. We
could add a BUG() there but I preferred to comment the offending function.

This was taken out from one Ingo Molnar's old patch I'm resurrecting.

References: commit b103e8b204b317d52834671d5f09db95645523c2 of old-2.6-bkcvs,
pointing to BKrev: 3f5ed0c1llm6NnNwNXtPv-Z0IYzkwA
