Direct syscalls using syscall.Syscall(SYS_*, ...) should no longer be
used on darwin, see [1]. Instead, use the fcntl libSystem wrappers
provided by the golang.org/x/sys/unix package which implement the same
functionality.
[1] https://golang.org/doc/go1.12#darwin
ftruncate changes st_blocks, and following fallocate
syscalls would return EINVAL when allocated block size
is already greater than requested block size
(e.g. st_blocks==8, requested blocks are 2).
Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>