jbd2: use str_plural() to fix Coccinelle warning

Fixes the following Coccinelle/coccicheck warning reported by
string_choices.cocci:

	opportunity for str_plural(dropped)

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Link: https://patch.msgid.link/20240402105157.254389-2-thorsten.blum@toblux.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>