Remove outdated comments and dead code from ext4_da_reserve_space.
Clean up its trace point, and relocate it to make it more useful.

While we're at it, fix a nearby conditional used to determine if
we have a non-bigalloc file system.  It doesn't match usage elsewhere
in the code, and misleadingly suggests that an s_cluster_ratio value
of 0 would be legal.
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>