vfs_io_uring: retry after a short read in vfs_io_uring_pread_completion()

vfs_io_uring: retry after a short read in vfs_io_uring_pread_completion()

We need to be prepared for short reads from the kernel depending on
the state of the page cache. Windows and Mac clients don't
expect short reads for files, so we need to retry ourself.

For the future we may be able to play with some io_uring flags
in order to avoid the retries in userspace, but for now we just fix
the data corruption bug...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14361

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>