fs/ntfs3: Fix fiemap + fix shrink file size (to remove preallocated space)

Two problems:
1. ntfs3_setattr can't truncate preallocated space;
2. if allocated fragment "cross" valid size, then fragment splits into two parts:
- normal part;
- unwritten part (here we must return FIEMAP_EXTENT_LAST).
Before this commit we returned FIEMAP_EXTENT_LAST for whole fragment.
Fixes xfstest generic/092
Fixes: 4342306f ("fs/ntfs3: Add file operations and implementation")

Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>