Move the ICL_CONN_*LOCK* macros to <dev/iscsi/icl.h>.

These macros are not backend-specific but reference a
backend-independent field in struct icl_conn.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32858

(cherry picked from commit e900338c0987603456df8d9a8aeec5c239106d0b)