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/DevPtsFS.cpp

@@ -12,9 +12,9 @@
 
 namespace Kernel {
 
-ErrorOr<NonnullRefPtr<DevPtsFS>> DevPtsFS::try_create()
+ErrorOr<NonnullRefPtr<FileSystem>> DevPtsFS::try_create()
 {
-    return adopt_nonnull_ref_or_enomem(new (nothrow) DevPtsFS);
+    return TRY(adopt_nonnull_ref_or_enomem(new (nothrow) DevPtsFS));
 }
 
 DevPtsFS::DevPtsFS() = default;