fiemap: fix fiemap size and log message when last extent

Fix the total fiemap struct plus extents size allocation.
Fix also the logging in IsLastExtent, that was printing error messages
every time.

Bug: none
Test: fiemap_*_test
Change-Id: I85c6af63ba3a75b993a5e8ad7d7170dc7af59582
Signed-off-by: Alessio Balsini <balsini@google.com>