fs/ntfs3: Use wrapper i_blocksize() in ntfs_zero_range()

Convert to use i_blocksize() for readability.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
[almaz.alexandrovich@paragon-software.com:
the patch has been partially accepted for performance reasons]
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>