erofs: sunset unneeded NOFAILs

With iterative development, our codebase can now deal with compressed
buffer misses properly if both in-place I/O and compressed buffer
allocation fail.

Note that if readahead fails (with non-uptodate folios), the original
request will then fall back to synchronous read, and `.read_folio()`
should return appropriate errnos; otherwise -EIO will be passed to
user space, which is unexpected.

To simplify rarely encountered failure paths, a mimic decompression
will be just used.  Before that, failure reasons are recorded in
compressed_bvecs[] and they also act as placeholders to avoid in-place
pages.  They will be parsed just before decompression and then pass
back to `.read_folio()`.

Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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