ubifs-utils: Add 'dev_fd' and 'libubi' into 'ubifs_info' structure

ubifs-utils: Add 'dev_fd' and 'libubi' into 'ubifs_info' structure

Embed new members 'dev_fd' and 'libubi' into ubifs_info structure, so
that global variable 'ubi', 'out_fd' and 'out_ubi' could be removed
from mkfs.ubifs.c. Besides, add parameter in check_volume_empty().
Next patch will extract UBI opening/closing/check_volume_empty functions
into a new source file, these functions will be used in fsck.

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