ubifs: Fix memory leak bug in alloc_ubifs_info() error path

In ubifs_mount(), 'c' is allocated through kzalloc() in alloc_ubifs_info().
However, it is not deallocated in the following execution if
ubifs_fill_super() fails, leading to a memory leak bug. To fix this issue,
free 'c' before going to the 'out_deact' label.

Fixes: 1e51764a ("UBIFS: add new flash file system")
Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Richard Weinberger <richard@nod.at>