[SPARC64] mm: update get_user_insn comment

Update comment on get_user_insn to the more general "pte lock", which may
or may not be the page_table_lock.  Note vmtruncate handled like kswapd.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>