ext4: make variable "count" signed

ext4: make variable "count" signed

Since dx_make_map() may return -EFSCORRUPTED now, so change "count" to
be a signed integer so we can correctly check for an error code returned
by dx_make_map().

Fixes: 617f37af334e ("ext4: verify dir block before splitting it")
Cc: stable@kernel.org
Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20220530100047.537598-1-dingxiang@cmss.chinamobile.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>