bcachefs: Make journal read log message a bit quieter

Users seem to be assuming that the 'dropped unflushed entries' message
at the end of journal read indicates some sort of problem, when it does
not - we expect there to be entries in the journal that weren't
commited, it's purely informational so that we can correlate journal
sequence numbers elsewhere when debugging.

Shorten the log message a bit to hopefully make this clearer.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>