common: Move exit related functions to a common/exit

common: Move exit related functions to a common/exit

Introduce a new file common/exit that will contain all the exit
related functions. This will remove the dependencies these functions
have on other non-related helper files and they can be indepedently
sourced. This was suggested by Dave Chinner[1].
While moving the exit related functions, remove _die() and die_now()
and replace die_now with _fatal(). It is of no use to keep the
unnecessary wrappers.

[1] https://lore.kernel.org/linux-xfs/Z_UJ7XcpmtkPRhTr@dread.disaster.area/

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>