Set st_nlink in O(1)

[overheadhunter]

While profiling for issue #33 I noticed, that getattr of directories (which happens frequently, e.g. when the caller wants to know if a file exists or is a directory) takes a lot of time due to nlinks depending on the number of subdirectories:

fuse-nio-adapter/src/main/java/org/cryptomator/frontend/fuse/ReadOnlyDirectoryHandler.java

Lines 43 to 50 in a412b13

	long nlinks;
	try {
	attrUtil.copyBasicFileAttributesFromNioToFuse(attrs, stat);
	nlinks = 2 + countSubDirs(path);
	} catch (IOException e) {
	nlinks = 2;
	}
	stat.st_nlink.set(nlinks);

The run time of countSubDirs vastly depends on the number of items in a directory, which makes getattr of the directory dependend on its size, which can lead to significant slowdowns:

Other FUSE file systems don't support link count either, such as sshfs, which simply hard-coded st_nlink=1.

Citing Han-Wen Nienhuys:

The st_nlink for dirs is actualy not well defined. But the convention under unix is that it is the number of subdirectories including "." and "..".

[...]

But I think st_nlink == 1 is save in that regard and find will see that st_nlink doesn't follow the unix convention. Any other st_nlink will need "find -noleaf".

While we should try to set st_nlink to the number of subdirectories + 2, it should be ok to hard-code a fixed value in case we can't find an efficient solution to get the number of subdirectories.

[overheadhunter]

Setting st_nlink = 1 works fine on and FUSE 2.9.7 (tested on Linux 4.15, Mint 19.1) as well as on FUSE for macOS 3.8.2 (tested on High Sierra)