coda: add hinting support for partial file caching

This adds support for partial file caching in Coda.  Every read, write
and mmap informs the userspace cache manager about what part of a file
is about to be accessed so that the cache manager can ensure the
relevant parts are available before the operation is allowed to proceed.

When a read or write operation completes, this is also reported to allow
the cache manager to track when partially cached content can be
released.

If the cache manager does not support partial file caching, or when the
entire file has been fetched into the local cache, the cache manager may
return an EOPNOTSUPP error to indicate that intent upcalls are no longer
necessary until the file is closed.

[akpm@linux-foundation.org: little whitespace fixup]
Link: http://lkml.kernel.org/r/20190618181301.6960-1-jaharkes@cs.cmu.edu
Signed-off-by: Pedro Cuadra <pjcuadra@gmail.com>
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>