LU-10141 llapi: integer overflow in llapi_changelog_start

LU-10141 llapi: integer overflow in llapi_changelog_start

Use the appropriate type to store and check the return value from lseek.
This prevents from misinterpreting high offsets as errors.

Change-Id: I15e92be3454af20ee6611c2a7ddfc1b597d639c2
Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-on: https://review.whamcloud.com/29674
Tested-by: Jenkins
Reviewed-by: Thomas LEIBOVICI <thomas.leibovici@cea.fr>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Stephan Thiell <sthiell@stanford.edu>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>