dm log: userspace fix overhead_size calcuations

dm log: userspace fix overhead_size calcuations

This patch fixes two bugs that revolve around the miscalculation and
misuse of the variable 'overhead_size'.  'overhead_size' is the size of
the various header structures used during communication.

The first bug is the use of 'sizeof' with the pointer of a structure
instead of the structure itself - resulting in the wrong size being
computed.  This is then used in a check to see if the payload
(data_size) would be to large for the preallocated structure.  Since the
bug produces a smaller value for the overhead, it was possible for the
structure to be breached.  (Although the current users of the code do
not currently send enough data to trigger this bug.)

The second bug is that the 'overhead_size' value is used to compute how
much of the preallocated space should be cleared before populating it
with fresh data.  This should have simply been 'sizeof(struct cn_msg)'
not overhead_size.  The fact that 'overhead_size' was computed
incorrectly made this problem "less bad" - leaving only a pointer's
worth of space at the end uncleared.  Thus, this bug was never producing
a bad result, but still needs to be fixed - especially now that the
value is computed correctly.

Cc: stable@kernel.org
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com
Signed-off-by: Alasdair G Kergon <agk@redhat.com>