iov_iter: use bvec iterator to implement iterate_bvec()

iov_iter: use bvec iterator to implement iterate_bvec()

bvec has one native/mature iterator for long time, so not
necessary to use the reinvented wheel for iterating bvecs
in lib/iov_iter.c.

Two ITER_BVEC test cases are run:
	- xfstest(-g auto) on loop dio/aio, no regression found
	- swap file works well under extreme stress(stress-ng --all 64 -t
	  800 -v), and lots of OOMs are triggerd, and the whole
	system still survives

Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Tested-by: Hannes Reinecke <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>