UBIFS: lessen the size of debugging info data structure

This patch lessens the 'struct ubifs_debug_info' size by 90 bytes by
allocating less bytes for the debugfs root directory name. It introduces macros
for the name patter an length instead of hard-coding 100 bytes. It also makes
UBIFS use 'snprintf()' and teaches it to gracefully catch situations when the
name array is too short.

Additionally, this patch makes 2 unrelated changes - I just thought they do not
deserve separate commits: simplifies 'ubifs_assert()' for non-debugging case
and makes 'dbg_debugfs_init()' properly verify debugfs return code which may be
an error code or NULL, so we should you 'IS_ERR_OR_NULL()' instead of
'IS_ERR()'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>