exofs: don't leak io_state and pages on read error

Same bug as fixed by Idan for write_exec was in read_exec.
Fix the io_state leak and pages state on read error.

Also while at it:
The if (!pcol->read_4_write) at the error path is redundant
because all goto err; are after the if (pcol->read_4_write)
bale out.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>