Add sizelimit to internal API

Add sizelimit to internal API

Fully safe checks of loop device need to check sizelimit. To prevent need of two
nearly equal functions, introduce sizelimit parameter to several internal
functions:
loopdev_is_used()
loopdev_find_by_backing_file()
loopcxt_is_used()
loopcxt_find_by_backing_file()

If sizelimit is zero, fall back to the old behavior (ignoring of sizelimit).

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>