Mark Linux fallocate extensions as specific to Linux

fallocate(2) is a Linux-specific system call which in unavailable
on other platforms.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9633

lib/libzpool/kernel.c

@@ -1346,7 +1346,11 @@ zfs_file_fsync(zfs_file_t *fp, int flags)
 int
 zfs_file_fallocate(zfs_file_t *fp, int mode, loff_t offset, loff_t len)
 {
+#ifdef __linux__
 	return (fallocate(fp->f_fd, mode, offset, len));
+#else
+	return (EOPNOTSUPP);
+#endif
 }
 
 /*