fs, net: Move read_descriptor_t to net.h

fs.h has no more need for this typedef; networking is now the sole user
of the read_descriptor_t.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>