libext2fs: improve jbd_debug() implementation

libext2fs: improve jbd_debug() implementation

Make jbd_debug() do format string checking (but still get compiled away
to nothing) when --enable-jbd-debug isn't specified, similar to
commit d556435156b7 ("jbd2: avoid -Wempty-body warnings") on the kernel
side.  This should prevent --enable-jbd-debug from getting broken due to
bad jbd_debug() statements.  It also eliminates a -Wunused-variable
warning where a variable was only used in a jbd_debug() statement.

Also remove an alternative definition of jbd_debug() that was
conditional on CONFIG_JBD_DEBUG && !CONFIG_JBD_DEBUG, so was dead code.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>