osd: fix merge_log missing calculation at split point when logs diverge

osd: fix merge_log missing calculation at split point when logs diverge

If primary log is something like

 1'1
 1'2
 1'3

and remote log is

 1'1
 1'2
 2'3
 2'4
 2'5

Then 1'2 is the split point, but we should advance p because it exists in both
logs.  Comparing to log.head is not a sufficient check for that.

Signed-off-by: Henry Chang <henry.cy.chang@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>