linux: fix linux_recvmsg() MSG_PEEK flag handling

Reviewed by: kib
Approved by: re(gjb)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33790

(cherry picked from commit 96c524d8b2c9abe3927abf948fbbd73a7cb42c4a)
(cherry picked from commit 0b6161db7eff92a37bc6d410ff9c9d5c3ac6f443)