fs/ntfs3: Remove tmp var is_ro in ntfs_fill_super

We only use this in two places so we do not really need it. Also
wrapper sb_rdonly() is pretty self explanatory. This will make little
bit easier to read this super long variable list in the beginning of
ntfs_fill_super().

Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>