An overflow may occur if the function is called with the last
block and an offset greater than zero. It is necessary to add
a check to avoid this.

Found by Linux Verification Center (linuxtesting.org) with Svace.

[JK: Make test cover also unalloc table freeing]

Link: https://patch.msgid.link/20240620072413.7448-1-r.smirnov@omp.ruSuggested-by: Jan Kara <jack@suse.com>
Signed-off-by: Roman Smirnov <r.smirnov@omp.ru>
Signed-off-by: Jan Kara <jack@suse.cz>