fallocate: fix FALLOC_FL_ZERO_RANGE flag check

Reported-by: Bernhard Voelker <mail@bernhard-voelker.de>
Signed-off-by: Karel Zak <kzak@redhat.com>
fallocate: make man page readable for humans

Well, let's copy & past from fallocate(2) syscall man page rather than
try to be creative with another description for the flags.

Reported-by: Bernhard Voelker <mail@bernhard-voelker.de>
Signed-off-by: Karel Zak <kzak@redhat.com>