Revert "net/socket: convert sock_map_fd() to FD_ADD()"

Revert "net/socket: convert sock_map_fd() to FD_ADD()"

This reverts commit 245f0d1c622b0183ce4f44b3e39aeacf78fae594.

When allocating a file sock_alloc_file() consumes the socket reference
unconditionally which isn't correctly handled in the conversion. This
can be fixed by massaging this appropriately but this is best left for
next cycle.

Reported-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/CADvbK_ewub4ZZK-tZg8GBQbDFHWhd9a48C+AFXZ93pMsssCrUg@mail.gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>