fs/ntfs: Use a helper function to access attributes

fs/ntfs: Use a helper function to access attributes

Right now to access the next attribute the code reads the length of the
current attribute and adds that to the current pointer. This is error
prone as bounds checking needs to be performed all over the place. So,
implement a helper and ensure its used across find_attr() and read_attr().

This commit does *not* implement full bounds checking. It is just the
preparation work for this to be added into the helper.

Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>