afs: Handle better the server returning excess or short data

When an AFS server is given an FS.FetchData{,64} request to read data from
a file, it is permitted by the protocol to return more or less than was
requested.  kafs currently relies on the latter behaviour in readpage{,s}
to handle a partial page at the end of the file (we just ask for a whole
page and clear space beyond the short read).

However, we don't handle all cases.  Add:

 (1) Handle excess data by discarding it rather than aborting.  Note that
     we use a common static buffer to discard into so that the decryption
     algorithm advances the PCBC state.

 (2) Handle a short read that affects more than just the last page.

Note that if a read comes up unexpectedly short of long, it's possible that
the server's copy of the file changed - in which case the data version
number will have been incremented and the callback will have been broken -
in which case all the pages currently attached to the inode will be zapped
anyway at some point.

Signed-off-by: David Howells <dhowells@redhat.com>