staging: erofs: fix conditional uninitialized `pcn' in z_erofs_map_blocks_iter

This patch adds error handling code for
z_erofs_map_blocks_iter to fix the compiler blame.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>