block: check for proper length of iov entries earlier in blk_rq_map_user_iov()

block: check for proper length of iov entries earlier in blk_rq_map_user_iov()

commit 5478755616ae2ef1ce144dded589b62b2a50d575 upstream.

commit 9284bcf checks for proper length of iov entries in
blk_rq_map_user_iov(). But if the map is unaligned, kernel
will break out the loop without checking for the proper length.
So we need to check the proper length before the unalign check.

Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>