coredump: cleanup coredump socket functions

We currently use multiple CONFIG_UNIX guards. This looks messy and makes
the code harder to follow and maintain. Use a helper function
coredump_sock_connect() that handles the connect portion. This allows us
to remove the CONFIG_UNIX guard in the main do_coredump() function.

Link: https://lore.kernel.org/20250605-schlamm-touren-720ba2b60a85@brauner


Signed-off-by: Christian Brauner <brauner@kernel.org>