mm/hmm: remove HMM_PFN_READ flag and ignore peculiar architecture

mm/hmm: remove HMM_PFN_READ flag and ignore peculiar architecture

Only peculiar architecture allow write without read thus assume that any
valid pfn do allow for read.  Note we do not care for write only because
it does make sense with thing like atomic compare and exchange or any
other operations that allow you to get the memory value through them.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jérôme Glisse <[email protected]>
Reviewed-by: John Hubbard <[email protected]>
Cc: Evgeny Baskakov <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Mark Hairgrove <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>