Documentation: freefall: simplify pathnames

Copying to local variable is actually not neccessary, if all we need
to do is snprintf(). This also removes problem where devname could be
missing zero termination.

Reported-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>