ntfs3: Restore NULL folio initialization in ntfs_writepages()

Clang warns (or errors with CONFIG_WERROR=y):

fs/ntfs3/inode.c:1021:6: error: variable 'folio' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
1021 | if (is_resident(ni)) {
| ^~~~~~~~~~~~~~~
fs/ntfs3/inode.c:1024:48: note: uninitialized use occurs here
1024 | while ((folio = writeback_iter(mapping, wbc, folio, &err)))
| ^~~~~

folio should be initialized to NULL for the first iteration of
writeback_iter() to start the loop properly. Restore the NULL
initialization of folio that was lost in the recent iomap conversion to
clear up the warning.

Fixes: 099ef9a ("fs/ntfs3: implement iomap-based file operations")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/oe-kbuild-all/202601010644.FIhOXy6Y-lkp@intel.com/
Closes: https://lore.kernel.org/r/202601010513.axd56bks-lkp@intel.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
[almaz.alexandrovich@paragon-software.com: added a few more tags]
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>