Fix unused variable warning in icl_soft.c

With clang 15, the following -Werror warning is produced:

sys/dev/iscsi//icl_soft.c:886:6: error: variable 'coalesced' set but not used [-Werror,-Wunused-but-set-variable]
        int coalesced, error;
            ^

The 'coalesced' variable is eventually used only in an #if 0'd block,
obviously meant for debugging. Ensure that 'coalesced' is only declared
and used when DEBUG_COALESCED is defined, so the debugging can be easily
turned on later, if desired.

MFC after: 3 days

(cherry picked from commit f4f847018048ac7699b55bc1915a393eb65e4c53)