afs: Do not limit fetches based on vcache length

Currently, when we go to the fileserver to fetch some data, we try to
make sure that we do not ask for data beyond the end of the file. For
example, if our chunk size is 1M, and we need to get the first chunk
for a file that is 4 bytes long, we will only ask the fileserver for 4
bytes.

This can cause issues when the file is being extended at the same time
as when we are trying to read the file. Consider the following
example. There is a file named X that has contents "abcd" at dv 1, and
we issue a FetchData64 request for X, only requesting 4 bytes. Right
before the fileserver gets the FetchData64 request, another client
writes the contents "12345" to file X.

The client will then fetch the contents "1234" for that file, at dv 2,
and store that as the contents of the first chunk for file X. On
subsequent reads for file X, applications will now get "1234<NUL>" as
the contents, since the size of the file will be updated to 5, but the
cache manager thinks that "1234" is the correct contents for the first
chunk of X at dv 2. The cache manager will continue to think so until
the cache entry is evicted or invalidated for whatever reason.

To avoid this scenario, always request a full chunk of data if we have
any data to fetch and the file has not been locally truncated. We can
still avoid the fetch at all if it looks like we're fetching beyond
end-of-file, since we know that at least at some point that was
correct information about the file. If this results in us trying to
fetch beyond end-of-file, the fileserver will respond with the correct
length anyway.

We still need to restrict the fetch request length based on
avc->f.truncPos, since the dcache data after avc->f.truncPos needs to
stay empty, since we don't track truncated data any other way. If we
also avoided this restriction, extending a file via truncation after
reducing a file's length via truncation could cause the old file data
to appear again, instead of filling the new file range with NULs.

Note that on at least Linux, with this fix an application can still
read the contents "1234" on the first read in the above example, and
"12345" on subsequent reads. This is just due to when we give the VFS
updates about file metadata, and could be remedied by updating file
metadata immediately from the FetchStatus information from the
FetchData64 call. However, just reading the contents "1234" in the
above example seems like a somewhat plausible outcome; at the very
least, it is an improvement.

Reviewed-on: http://gerrit.openafs.org/6882
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit e53221d9a82fd8e3d545704abae51cc844bc31a3)

Change-Id: I81b5a3a6ff745f3f53988a1a4e5d3df20f5df6d3
Reviewed-on: http://gerrit.openafs.org/7994
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
afs: Slight adjustments in afs_GetDCache

maxGoodSize is only used in one block, so move the decl to that block.
Adjust some of the comments to more accurately reflect what's going
on.

Reviewed-on: http://gerrit.openafs.org/6936
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit d5c13a0f3ce7d9f0a01820ff9c12dfb1cbc12047)

Change-Id: I2ba6574b88856d2f506663eafb984a9f3bea4cf8
Reviewed-on: http://gerrit.openafs.org/7995
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>