btrfs: print target number of bytes when dumping free space

When dumping free space, with btrfs_dump_free_space(), we pass a bytes
argument in order to count how many free space entries in the block group
have a size greater than or equal to that number of bytes. We then print
how many suitable entries we found, but we don't print the target number
of bytes, we just say "bytes". Change the message to actually print the
number of bytes, which makes debugging -ENOSPC issues a bit easier.

Also sligthly change the odd grammar and terminology: the sentence is
ending with 'is', which doesn't make sense, and the term 'blocks' is
confusing as we are referring to free space entries within the block
group's free space cache.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>