NTFS: - Fix endianness bug in ntfs_external_attr_find().

NTFS: - Fix endianness bug in ntfs_external_attr_find().
      - Change ntfs_{external_,}attr_find() to return 0 on success, -ENOENT
	if the attribute is not found, and -EIO on real error.  In the case
	of -ENOENT, the search context is updated to describe the attribute
	before which the attribute being searched for would need to be
	inserted if such an action were to be desired and in the case of
	ntfs_external_attr_find() the search context is also updated to
	indicate the attribute list entry before which the attribute list
	entry of the attribute being searched for would need to be inserted
	if such an action were to be desired.  Also make ntfs_find_attr()
	static and remove its prototype from attrib.h as it is not used
	anywhere other than attrib.c.  Update ntfs_attr_lookup() and all
	callers of ntfs_{external,}attr_{find,lookup}() for the new return
	values.
      - Force use of ntfs_attr_find() in ntfs_attr_lookup() when searching
	for the attribute list attribute itself.

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