fs: Fix fs_open resource leak when invoked on fs_file_t object in use

Fixes problem when fs_open invoked on fs_file_t object, which is already
holding information on opened file, overwrites references to other
memory objects within the fs_file_t object causing resource leak.
If fs_open is invoked on already used fs_file_t object, it will return
-EBUSY.

Note: The change requires that all fs_file_t objects should be
initialized to 0.

Fixes: #29478

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>