Staging: ramzswap: Handle case for invalid backing swap

Currently, we crash (issue BUG_ON) if backing swap
disk size is zero. This can happen is user specified
an extended partition or simply a bad disk as backing
swap. A crash is really an unpleasant surprise to user
for such trivial problems.

Now, we check for this condition and simply fail device
initialization if this is the case.

Additional cleanups:
 * use static for all functions
 * remove extra newline between functions
 * memset backing_swap_name to NULL on device reset

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>