FileJournal: fix check_for_full

FileJournal: fix check_for_full

The condition to test if journal is passing half full mark is
incorrect. Consider the following simplified case:

Assume header.max_size = 100 and journal has 48 bytes already.

1st write (10 bytes):
max=100, room=52, size=10  --> condition not satisfied

2nd write (4 bytes):
max=100, room=42, size=4   --> condition not satisfied

It passed the half full mark without triggering filestore commit.

Signed-off-by: Henry Chang <henry@bigtera.com>