libc: posix_fallocate must return an error number on failure

libc: posix_fallocate must return an error number on failure

posix_fallocate implementation in uClibc relies on fallocate
system call - it just returns what fallocate returns. However
fallocate returns -1 on failure and assigns an error number
to errno variable. In the same time posix_fallocate must
return an error number but not -1.

What does this patch: if fallocate returns -1 then posix_fallocate
returns errno. Otherwise posix_fallocate returns 0 on success.

However there is a side effect - posix_fallocate sets errno on
failure because fallocate does it. But POSIX does not forbid it
thus it's not a problem.

Signed-off-by: Yuriy Kolerov <yuriy.kolerov@synopsys.com>