ubifs-utils: Move 'debug_level' into ubifs_info structure

ubifs-utils: Move 'debug_level' into ubifs_info structure

Embed new member 'debug_level' into 'ubifs_info' structure, then global
variable 'debug_level' can be removed from mkfs.ubifs.c. Next patches
will import UBIFS libs from linux kernel, which could print messages
with different levels, then 'debug_level' can be used to control which
level messages should be printed.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>