NTFS: Fix several bugs in fs/ntfs/attrib.c.

- Fix a bug in ntfs_map_runlist_nolock() where we forgot to protect
  access to the allocated size in the ntfs inode with the size lock.
- Fix ntfs_attr_vcn_to_lcn_nolock() and ntfs_attr_find_vcn_nolock() to
  return LCN_ENOENT when there is no runlist and the allocated size is
  zero.
- Fix load_attribute_list() to handle the case of a NULL runlist.

Signed-off-by: Anton Altaparmakov <aia21@cantab.net>