[DFSan] Fix recvmsg wrapper to support MSG_TRUNC flag. (#92599)

The MSG_TRUNC flag makes recvmsg return the real length of the packet,
even if it was too big to fit in the provided buffer. This is commonly
used together with MSG_PEEK.

Without this patch, dfsan's clear_msghdr_labels expects the return value
of recvmsg (size recieved) to be less than or equal to the iov buffer
length where recvmsg writes data, resulting in a crash.