jbd2: Fix comment describing journal_init_common()

jbd2: Fix comment describing journal_init_common()

The code indicates that journal_init_common() fills the journal_t object
it returns while the comment incorrectly states that only a few fields are
initialised.  Also, the comment claims that journal structures could be
created from scratch which isn't possible as journal_init_common() calls
journal_load_superblock() which loads and checks journal superblock from
disk.

Signed-off-by: Daniel Martín Gómez <dalme@riseup.net>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20241107144538.3544-1-dalme@riseup.net
Signed-off-by: Theodore Ts'o <tytso@mit.edu>