fallocate: Hide #ifdef tricks to call fallocate in a function

fallocate: Hide #ifdef tricks to call fallocate in a function

Future patches will add more calls to fallocate(), so it will be useful to have
all these tricks inside a function.

The error message when fallocate is not supported is slightly changed: the file
name is not printed as a prefix because is not available in the context of the
function. Also, to only print one of the two possible errors (as happens when
using directly exit()), an else clause was added.

Signed-off-by: Rodrigo Campos <rodrigo@sdfg.com.ar>