NTFS: Rename {{re,}init,get,put}_attr_search_ctx() to

NTFS: Rename {{re,}init,get,put}_attr_search_ctx() to
      ntfs_attr_{{re,}init,get,put}_search_ctx() as well as the type
      attr_search_context to ntfs_attr_search_ctx.

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