readonly gateway fatel and other related problems after startup

[YunhuiChen]

What happened:

What you expected to happen:
The gateway readonly startup is supported and works properly

How to reproduce it (as minimally and precisely as possible):
Here are some of the issues encountered during testing：

1、 start gateway:
./juicefs gateway redis://127.0.0.1:6379/1 0.0.0.0:9001 --keep-etag --cache-size 0 --backup-meta 1d --buffer-size 4096 --subdir fstests --multi-buckets --read-only
./mc mb gateway1/test
gateway fatel :
runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0xc02266c398 stack=[0xc02266c000, 0xc04266c000]
fatal error: stack overflow

runtime stack:
runtime.throw({0x30ba74b?, 0x7f7b6f69ddd8?})
/usr/local/go/src/runtime/panic.go:1077 +0x5c fp=0x7f7b6f69dcc0 sp=0x7f7b6f69dc90 pc=0x44e13c
runtime.newstack()
/usr/local/go/src/runtime/stack.go:1107 +0x5ac fp=0x7f7b6f69de70 sp=0x7f7b6f69dcc0 pc=0x468c2c
runtime.morestack()
/usr/local/go/src/runtime/asm_amd64.s:593 +0x8f fp=0x7f7b6f69de78 sp=0x7f7b6f69de70 pc=0x48312f

goroutine 44 [running]:
runtime.mallocgc(0x2?, 0x0?, 0x0?)
/usr/local/go/src/runtime/malloc.go:948 +0x825 fp=0xc02266c3a8 sp=0xc02266c3a0 pc=0x423265
runtime.rawstring(...)
/usr/local/go/src/runtime/string.go:267

2、readonly gateway Cannot be operated by a non-root account

3、The web console does not report any error when deleting data or creating directories
4、Metadata cannot be backed up under readonly. warning information is required

Anything else we need to know?

Environment:

• JuiceFS version (use juicefs --version) or Hadoop Java SDK version:
• Cloud provider or hardware configuration running JuiceFS:
• OS (e.g cat /etc/os-release):
• Kernel (e.g. uname -a):
• Object storage (cloud provider and region, or self maintained):
• Metadata engine info (version, cloud provider managed or self maintained):
• Network connectivity (JuiceFS to metadata engine, JuiceFS to object storage):
• Others: