submodule--helper.c: Rename 'cb_foreach' to 'foreach_cb'

submodule--helper.c: Rename 'cb_foreach' to 'foreach_cb'

In 'submodule--helper.c', the structures and macros for callbacks belonging
to any subcommand are named in the format: 'subcommand_cb' and 'SUBCOMMAND_CB_INIT'
respectively.

This was an exception for the subcommand 'foreach' of the command
'submodule'. Rename the aforementioned structures and macros:
'struct cb_foreach' to 'struct foreach_cb' and 'CB_FOREACH_INIT'
to 'FOREACH_CB_INIT'.

Signed-off-by: Shourya Shukla <shouryashukla.oo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>