dm: fix failure when empty flush's bi_sector points beyond the device end

dm: fix failure when empty flush's bi_sector points beyond the device end

An empty flush bio can have arbitrary bi_sector. The commit 2b1c6d7a890a
introduced a regression that device mapper would fail an empty flush bio
with -EIO if the sector pointed beyond the end of the device.

The commit introduced an optimization, that optimization would pass
flushes to __split_and_process_bio and __split_and_process_bio is not
prepared to handle empty bios. Fix this bug by passing only non-empty
flushes to __split_and_process_bio - non-empty flushes must have valid
bi_sector. Empty bios will go through __send_empty_flush, as they did
before the optimization.

This problem can be reproduced by running the lvm2 test:
make check_local T=lvconvert-thin.sh LVM_TEST_PREFER_BRD=0

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 2b1c6d7a890a ("dm: optimize REQ_PREFLUSH with data when using the linear target")
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>