generic: read multiple extents with io_uring

generic: read multiple extents with io_uring

Test doing a read, with io_uring, over a file range that includes
multiple extents. The read operation triggers page faults when
accessing all pages of the read buffer except for the pages
corresponding to the first extent.  We want to check that the
operation results in reading all the extents and that it returns the
correct data.

This is motivated by an issue found with MariaDB when using io_uring
and running on btrfs. There's a patch for btrfs to address the issue

ca93e44bfb5f ("btrfs: fallback to blocking mode when doing async dio
over multiple extents")

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>