Instead of using pre-allocated 'ubi->dbg_peb_buf' buffer in 'ubi_dbg_check_write()', dynamically allocate it when needed. The intend is to get rid of the pre-allocated 'ubi->dbg_peb_buf' buffer completely. And the need for this arises because we want to change to dynamic debugging control instead of compile-time control, i.e., we are going to kill the CONFIG_MTD_UBI_DEBUG_PARANOID Kconfig option, which would mean that 'ubi->dbg_peb_buf' is always allocated, which would be wasteful. Thus, we are getting rid of 'ubi->dbg_peb_buf', and this is a preparation for that. Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>