Kernel: Don't allocate Strings unnecessarily in process veil validation

Previously, Custody::absolute_path() was called for every call to
validate_path_against_process_veil(). For processes that don't have a
veil, the path is not used by the function. This means that it is
unnecessarily generated. This introduces an overload to
validate_path_against_process_veil(), which takes a Custody const& and
only generates the absolute path if it there is actually a veil and it
is thus needed.

This patch results in a speed up of Assistant's file system cache
building by around 16 percent.

Kernel/FileSystem/VirtualFileSystem.h

@@ -103,6 +103,7 @@ private:
     friend class FileDescription;
 
     UnveilNode const& find_matching_unveiled_path(StringView path);
+    KResult validate_path_against_process_veil(Custody const& path, int options);
     KResult validate_path_against_process_veil(StringView path, int options);
 
     bool is_vfs_root(InodeIdentifier) const;