Fixes for the LinuxAPI completion wrappers:

• make sure the timeout computations are always above zero by using

the existing "linux_timer_jiffies_until()" function. Negative timeouts
can result in undefined behaviour.

• declare all completion functions like external symbols and move the

code to the LinuxAPI kernel module.

• add a proper prefix to all LinuxAPI kernel functions to avoid

namespace collision with other parts of the FreeBSD kernel.

• clean up header file inclusions in the linux/completion.h, linux/in.h

and linux/fs.h header files.

MFC after: 1 week
Sponsored by: Mellanox Technologies