fallocate: fix check of number of arguments

Signed-off-by: Bernhard Voelker <mail@bernhard-voelker.de>
fallocate: use O_CREAT only for the default behavior

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