vfs: fix vmplice_to_user()

Commit 6130f531

 "switch vmsplice_to_user() to copy_page_to_iter()" in
v3.15-rc1 broke vmsplice(2).

This patch fixes two bugs:

 - count is not initialized to a proper value, which resulted in no data
   being copied

 - if rw_copy_check_uvector() returns negative then the iov might be leaked.

Tested OK.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>