MFC r329825:
Return correct error code to user-space when a system call receives a
signal in the LinuxKPI.

The read(), write() and mmap() system calls can return either EINTR or
ERESTART upon receiving a signal. Add code to figure out the correct
return value by temporarily storing the return code from the relevant
FreeBSD kernel APIs in the Linux task structure.

Sponsored by: Mellanox Technologies