r217592 moved the check for imo in udp_input() into the conditional block
but leaving the variable assignment outside the block, where it is no longer
used. Move both the variable and the assignment one block further in.

This should result in no functional changes. It will however make upcoming
changes slightly easier to apply.

Reviewed by: markj, jtl, tuexen
Approved by: re (kib)
Differential Revision: https://reviews.freebsd.org/D17525