UBIFS: do not treat ENOSPC specially

'ubifs_garbage_collect_leb()' should never return '-ENOSPC', and if it
does, this is an error. Thus, do not treat this error code specially.
'-EAGAIN' is a special error code, but not '-ENOSPC'.

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