libdevs: make the safe device bitmap word unsigned

[mminkus]

SDEV_BITMAP_WORD is long, so sdev_is_block_saved() and sdev_mark_blocks()
evaluate (long)1 << 63 whenever a block position is congruent to 63 modulo 64.
Shifting into the sign bit of a signed type is undefined behaviour.

In practice gcc produces the intended bit pattern today, so the bitmap happens to
work. It is still UB, and the bitmap is what the safe device uses to decide
whether a block has already been saved, so a wrong answer there would either lose
an original block or consume the saved block budget twice.

Reproducing

Build with UBSan and probe a simulated device:

make build/f3probe \
  CFLAGS="-std=c17 -Wall -Wextra -pedantic -MMD -ggdb -fsanitize=undefined" \
  LDFLAGS="-fsanitize=undefined"

./build/f3probe --debug --debug-real-size=264241152 \
  --debug-fake-size=67108864000 --debug-wrap=36 /tmp/sim

src/libdevs.c:1158:32: runtime error: left shift of 1 by 63 places cannot be
    represented in type 'long int'
src/libdevs.c:1170:51: runtime error: left shift of 1 by 63 places cannot be
    represented in type 'long int'

Verifying the fix

I swept 63 simulated geometries, varying real size, announced size and wrap.
Every run reported the shift before this change; none do after it, and every
probed size is identical either way, so this is a pure UB fix with no behaviour
change.

Found while probing a counterfeit card on aarch64, Debian 13, gcc 14.2.

[AltraMayor]

Thank you for this review, @mminkus.