fs/ntfs3: Refactor enum_rstbl to suppress static checker

Comments and brief description of function enum_rstbl added.

Fixes: b46acd6a ("fs/ntfs3: Add NTFS journal")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>