gadget: remove only user of aio retry

This removes the only in-tree user of aio retry.  This will let us
remove the retry code from the aio core.

Removing retry is relatively easy as the USB gadget wasn't using it to
retry IOs at all.  It always fully submitted the IO in the context of
the initial io_submit() call.  It only used the AIO retry facility to
get the submitter's mm context for copying the result of a read back to
user space.  This is easy to implement with use_mm() and a work struct,
much like kvm does with async_pf_execute() for get_user_pages().

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Zach Brown <zab@redhat.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Asai Thambi S P <asamymuthupa@micron.com>
Cc: Selvan Mani <smani@micron.com>
Cc: Sam Bradshaw <sbradshaw@micron.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>