Kernel/FileSystem: Simplify even more the mount syscall

As with the previous commit, we put a distinction between filesystems
that require a file description and those which don't, but now in a much
more readable mechanism - all initialization properties as well as the
create static method are grouped to create the FileSystemInitializer
structure. Then when we need to initialize an instance, we iterate over
a table of these structures, checking for matching structure and then
validating the given arguments from userspace against the requirements
to ensure we can create a valid instance of the requested filesystem.

Kernel/FileSystem/SysFS.cpp

@@ -68,9 +68,9 @@ SysFSRootDirectory::SysFSRootDirectory()
     m_buses_directory = buses_directory;
 }
 
-ErrorOr<NonnullRefPtr<SysFS>> SysFS::try_create()
+ErrorOr<NonnullRefPtr<FileSystem>> SysFS::try_create()
 {
-    return adopt_nonnull_ref_or_enomem(new (nothrow) SysFS);
+    return TRY(adopt_nonnull_ref_or_enomem(new (nothrow) SysFS));
 }
 
 SysFS::SysFS() = default;