The roundup() and rounddown() macros return the rounded value but
do not modify the input in place. In _badblocks_set(), _badblocks_clear(),
and badblocks_check(), the return values were being discarded, causing
s and target/next to remain unrounded. This resulted in sectors
being calculated from unrounded values, which could lead to sectors
being way too large (or zero), causing infinite loops in the
re_insert/re_clear/re_check loops.

Additionally, add integer overflow checks (s > ULLONG_MAX - sectors)
before the s + sectors calculation in all three functions to prevent
overflow-related issues. Also add early return when sectors becomes
zero after rounding in badblocks_check().

Root cause: When s and sectors have specific values (e.g., from
syzkaller fuzzing via nvdimm ioctl), the unrounded values cause
sectors to be incorrectly calculated. In _badblocks_clear(), this
could result in needing 2^46 iterations to process 2^55 sectors,
triggering RCU stall warnings and effectively hanging the kernel.

Fix by properly capturing the return values from roundup() and
rounddown(), adding overflow checks before sector arithmetic, and
handling the zero-sectors case in badblocks_check().

Signed-off-by: Ramesh Adhikari <adhikari.resume@gmail.com>