Introduce a fast unlink function for rmr feature

[anysql]

What would you like to be added:

Just check the source code, the rmr will call "Unlink" function for file deletion, it will update the parent directory attributes (nlink/mtime etc) and statistics (quota limits) for each file. Since we are going to drop the directory, there is no need for these extra steps, at least we can consider a batch pipeline mode for less network roundtrips.

Why is this needed:

Improve the rmr performance, may be 2x or more faster.

[davies]

We should update the quota, and skip some of operations on parent.

[anysql]

For Unlink operation, parent node is not always updated, reduce operations does not help much.

If we skip checking the following two steps for rmr emptyDir operation, no need to get the parent node info (save a roundtrip), which can improve the rmr performance by 10% (local test from 1737.7/s to 1955.1/s).

                    if (pn.Flags&FlagAppend) != 0 || (pn.Flags&FlagImmutable) != 0 {
                            return syscall.EPERM
                    }

                    if ctx.Uid() != 0 && pn.Mode&01000 != 0 && ctx.Uid() != pn.Uid && ctx.Uid() != n.Uid {
                            return syscall.EACCES
                    }

[anysql]

If we pass the edge info from emptyDir to skip the query of edge info, and also skip the xattr delete (usually empty).

The throughput can improved to 2326.2/s in local test.