libceph: only kunmap kmapped pages

In write_partial_msg_pages(), pages need to be kmapped in order to
perform a CRC-32c calculation on them.  As an artifact of the way
this code used to be structured, the kunmap() call was separated
from the kmap() call and both were done conditionally.  But the
conditions under which the kmap() and kunmap() calls were made
differed, so there was a chance a kunmap() call would be done on a
page that had not been mapped.

The symptom of this was tripping a BUG() in kunmap_high() when
pkmap_count[nr] became 0.

Reported-by: Bryan K. Wright <bryan@virginia.edu>
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>