afs: Maintain netfs_i_context::remote_i_size

Make afs use netfslib's tracking for the server's idea of what the current
inode size is independently of inode->i_size.  We really want to use this
value when calculating the new vnode size when initiating a StoreData RPC
op rather than the size stat() presents to the user (ie. inode->i_size) as
the latter is affected by as-yet uncommitted writes.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org

Link: https://lore.kernel.org/r/164623014626.3564931.8375344024648265358.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/164678220204.1200972.17408022517463940584.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/164692923592.2099075.5466132542956550401.stgit@warthog.procyon.org.uk/ # v3