LinuxKPI: Fix typo in cond_resched_lock

Lock must be released rather than acquired around mi_switch call.

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D35048

(cherry picked from commit 41559beb000555770cff1e1b3ef0275157aeb608)