ext4: Fix ext4_ext_journal_restart()

The ext4_ext_journal_restart() is a convenience function which checks
to see if the requested number of credits is present, and if so it
closes the current transaction and attaches the current handle to the
new transaction.  Unfortunately, it wasn't proprely checking the
return value from ext4_journal_extend(), so it was starting a new
transaction when one was not necessary, and returning an error when
all that was necessary was to restart the handle with a new
transaction.

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