ext4: minor defrag code improvements

ext4: minor defrag code improvements

Modify the error returns for two file types that can't be defragged to
more clearly communicate those restrictions to a caller.  When the
defrag code is applied to swap files, return -ETXTBSY, and when applied
to quota files, return -EOPNOTSUPP.  Move an extent tree search whose
results are only occasionally required to the site always requiring them
for improved efficiency.  Address a few typos.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20220722163910.268564-1-enwlinux@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>