erofs: silence uninitialized variable warning in z_erofs_scan_folio()

erofs: silence uninitialized variable warning in z_erofs_scan_folio()

Smatch complains that:

    fs/erofs/zdata.c:1047 z_erofs_scan_folio()
    error: uninitialized symbol 'err'.

The issue is if we hit this (!(map->m_flags & EROFS_MAP_MAPPED)) {
condition then "err" isn't set.  It's inside a loop so we would have to
hit that condition on every iteration.  Initialize "err" to zero to
solve this.

Fixes: 5b9654efb604 ("erofs: teach z_erofs_scan_folios() to handle multi-page folios")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/f78ab50e-ed6d-4275-8dd4-a4159fa565a2@stanley.mountain
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>