dm era: fix out-of-bounds memory access for non-zero start sector

dm era: fix out-of-bounds memory access for non-zero start sector

dm-era tracks writes in target-relative blocks, but era_map() calculates
the writeset block before applying the target offset.  Tables with a
non-zero start sector can therefore pass an absolute mapped-device block
to metadata_current_marked().

If the absolute block is beyond the current writeset size,
writeset_marked() tests past the end of the in-core bitset.  KASAN reports
this as a vmalloc-out-of-bounds access.

Apply the target offset before calculating the era block so writeset
lookups use the target-relative block number.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
Reviewed-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Fixes: eec40579d848 ("dm: add era target")