io_u: get io_u from io_u_freelist when TD_FSYNCING

io_u: get io_u from io_u_freelist when TD_FSYNCING

As Commit 813445e71292 ('backend: clean up requeued io_u's') has been
applied, backend cleans up the remained io_u's in td->io_u_requeues.
However, with end_fsync=1, the __get_io_u() function returns an io_u
from td->io_u_requeues if any io_u exist, and pops it. This leads that
the synced io_u will not put file which it got, and, finally, cannot
close the file.

This patch returns io_u from td->io_u_free_list when td->runstate is
TD_FSYNCING, so that the io_u's in td->io_u_requeues will be cleaned up
and leads to close file appropriately.

Signed-off-by: Jonghwi Jeong <jongh2.jeong@samsung.com>