fs: explicitly initialize fs_file_t and fs_dir_t

C++ compilers forbid usage of compound literals with -Wpedantic enabled.
They also forbid the usage of designated initializers for standards
lower than c++20.
So switch to explicit initialization, that would not make compilers
angry at us.

Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>