Fixes for the LinuxAPI completion wrappers:

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