curvefs/metaserver: cannot get a full copy of one inode

[h0hmj]

Describe the bug (描述bug)

if we want to get a copy of a inode,
we need GetInode and PaddingInodeS3ChunkInfo two rpc from InodeManager.

rpc1 -> GetInode: len=1 (s3chunkinfolist: slice 1)
changes -> some write, s3chunkinfolist: silce 1, slice 2
rpc2 -> PaddingInodeS3ChunkInfo: s3chunkinfolist: slice 1, slice 2

so we get inode, len=1 and s3chunkinfolist: slice1, slice2, which is wrong

if our s3compact threshold is 1, it will be compact to len=1, s3chunkinfolist: slice1
slice2 is regarded as useless data maybe from truncate smalle, and will be deleted.

inode will be updated to len=2, s3chunkinfolist: slice 1; we lose our data.

So we need a method to get a full copy of a inode when needed.

To Reproduce (复现方法)

Expected behavior (期望行为)

Versions (各种版本)
commit: c783e83

Additional context/screenshots (更多上下文/截图)