fscache: Implement raw I/O interface

Provide a pair of functions to perform raw I/O on the cache.  The first
function allows an arbitrary asynchronous direct-IO read to be made against
a cache object, though the read should be aligned and sized appropriately
for the backing device:

        int fscache_read(struct netfs_cache_resources *cres,
                         loff_t start_pos,
                         struct iov_iter *iter,
                         enum netfs_read_from_hole read_hole,
                         netfs_io_terminated_t term_func,
                         void *term_func_priv);

The cache resources must have been previously initialised by
fscache_begin_read_operation().  A read operation is sent to the backing
filesystem, starting at start_pos within the file.  The size of the read is
specified by the iterator, as is the location of the output buffer.

If there is a hole in the data it can be ignored and left to the backing
filesystem to deal with (NETFS_READ_HOLE_IGNORE), a hole at the beginning
can be skipped over and the buffer padded with zeros
(NETFS_READ_HOLE_CLEAR) or -ENODATA can be given (NETFS_READ_HOLE_FAIL).

If term_func is not NULL, the operation may be performed asynchronously.
Upon completion, successful or otherwise, (*term_func)() will be called and
passed term_func_priv, along with an error or the amount of data
transferred.  If the op is run asynchronously, fscache_read() will return
-EIOCBQUEUED.

The second function allows an arbitrary asynchronous direct-IO write to be
made against a cache object, though the write should be aligned and sized
appropriately for the backing device:

        int fscache_write(struct netfs_cache_resources *cres,
                          loff_t start_pos,
                          struct iov_iter *iter,
                          netfs_io_terminated_t term_func,
                          void *term_func_priv);

This works in very similar way to fscache_read(), except that there's no
need to deal with holes (they're just overwritten).

The caller is responsible for preventing concurrent overlapping writes.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819613224.215744.7877577215582621254.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906915386.143852.16936177636106480724.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967122632.1823006.7487049517698562172.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021521420.640689.12747258780542678309.stgit@warthog.procyon.org.uk/ # v4