filemap: fix the last_index of mm_filemap_get_pages

filemap: fix the last_index of mm_filemap_get_pages

In commit b6273b55d885 ("filemap: add trace events for get_pages,
map_pages, and fault"), mm_filemap_get_pages was added to trace page cache
access.  However, it tracks an extra page beyond the end of the accessed
range.  This patch fixes it by replacing last_index with last_index - 1.

Link: https://lkml.kernel.org/r/20240903102100.70405-1-takayas@chromium.org
Fixes: b6273b55d885 ("filemap: add trace events for get_pages, map_pages, and fault")
Signed-off-by: Takaya Saeki <takayas@chromium.org>
Cc: Junichi Uekawa <uekawa@chromium.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>