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] osxfuse/sshfs@5c0dbfe
[2] https://github.com/libfuse/sshfs/blob/4c21d696e9d46bebae0a936e2aec72326c5954ea/sshfs.c#L812
[3] #11 (comment)

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;