jffs2: Fix serious write stall due to erase

Drop the alloc_sem before erasing flash in
jffs2_garbage_collect_pass().
Otherwise writes are put on hold until the erase
has finised.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>