libfs: add simple_open()

libfs: add simple_open()

debugfs and a few other drivers use an open-coded version of simple_open()
to pass a pointer from the file to the read/write file ops.  Add support
for this simple case to libfs so that we can remove the many duplicate
copies of this simple function.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>