genfs_getpages() can read in more blocks than it should due to faked filesize
of lfs_gop_size(). it's a security problem and it makes gcc3 "internal error"
to fix this,
- in genfs_getpages(), always calculate diskeof and memeof separately
so that filesystems (in this case, lfs) can use different strategies
for them.
- introduce GOP_SIZE_MEM flag and use it to request in-core filesize.
(it was an intention of GOP_SIZE_READ,
but after the above change _READ is not a straightforward name)
after this, no one uses GOP_SIZE_{READ,WRITE} anymore but leave them for now.
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