jbd2: more simplifications in do_get_write_access()

Check for the simple case of unjournaled buffer first, handle it and
bail out. This allows us to remove one if and unindent the difficult case
by one tab. The result is easier to read.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>