sshfs: fix another instance preventing use of global I/O size on macOS (#185)

Following-up on [1], there was another instance where blksize
was set to a non-zero value, thus making it impossible to
configure global I/O size on macOS, and using [2] the hard-wired
value of 4096 bytes instead, resulting in uniformly poor
performance [3].

With this patch, setting I/O size to a reasonable large value,
will result in much improved performance, e.g.:
  -o iosize=1048576

[1] 5c0dbfe3eb
[2] 4c21d696e9/sshfs.c (L812)
[3] https://github.com/libfuse/sshfs/issues/11#issuecomment-339407557

sshfs.c

@@ -3920,7 +3920,11 @@ int main(int argc, char *argv[])
 	}
 #endif /* __APPLE__ */
 
+#ifdef __APPLE__
+	sshfs.blksize = 0;
+#else
 	sshfs.blksize = 4096;
+#endif
 	/* SFTP spec says all servers should allow at least 32k I/O */
 	sshfs.max_read = 32768;
 	sshfs.max_write = 32768;