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/ProcFS.cpp

@@ -37,9 +37,9 @@ UNMAP_AFTER_INIT ProcFSComponentRegistry::ProcFSComponentRegistry()
 {
 }
 
-ErrorOr<NonnullRefPtr<ProcFS>> ProcFS::try_create()
+ErrorOr<NonnullRefPtr<FileSystem>> ProcFS::try_create()
 {
-    return adopt_nonnull_ref_or_enomem(new (nothrow) ProcFS());
+    return TRY(adopt_nonnull_ref_or_enomem(new (nothrow) ProcFS));
 }
 
 ProcFS::ProcFS() = default;