[NTFS] - Restructure some code in preparation for the next commit:

[NTFS] - Restructure some code in preparation for the next commit:
-SetAttributeDataLength() has been split into two functions, SetNonResidentAttributeDataLength() and SetResidentAttributeDataLength(). This should improve code readibility and allows for resizing an attribute when there's no FileObject associated with it.
-Added "MftDataOffset" member to DEVICE_EXTENSION, which stores the offset of the Mft's $DATA attribute. (I'm starting to think it's better to add a member for offset to NTFS_ATTR_CONTEXT directly, but I'll save that level of restructuring for a future commit.)

svn path=/branches/GSoC_2016/NTFS/; revision=75055