net: handle error more gracefully in socketpair()

This patch makes socketpair() use error paths which do not
rely on heavy-weight call to sys_close(): it's better to try
to push the file descriptor to userspace before installing
the socket file to the file descriptor, so that errors are
catched earlier and being easier to handle.

Using sys_close() seems to be the exception, while writing the
file descriptor before installing it look like it's more or less
the norm: eg. except for code used in init/, error handling
involve fput() and put_unused_fd(), but not sys_close().

This make socketpair() usage of sys_close() quite unusual.
So it deserves to be replaced by the common pattern relying on
fput() and put_unused_fd() just like, for example, the one used
in pipe(2) or recvmsg(2).

Three distinct error paths are still needed since calling
fput() on file structure returned by sock_alloc_file() will
implicitly call sock_release() on the associated socket
structure.

Cc: David S. Miller <davem@davemloft.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Link: http://marc.info/?i=1385979146-13825-1-git-send-email-ydroneaud@opteya.com
Signed-off-by: David S. Miller <davem@davemloft.net>