sshfs: backport upstream commit

* This commit backports libfuse/sshfs commit 667cf34

Original commit message:

    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] libfuse/sshfs#11 (comment)

fuse/sshfs/files/sshfs.c.patch

@@ -10,3 +10,15 @@
  #include <assert.h>
  #include <stdio.h>
  #include <stdlib.h>
+@@ -3859,7 +3859,11 @@
+ }
+ #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;