ext4: move ext4_forget() to ext4_jbd2.c

The ext4_forget() function better belongs in ext4_jbd2.c.  This will
allow us to do some cleanup of the ext4_journal_revoke() and
ext4_journal_forget() functions, as well as giving us better error
reporting since we can report the caller of ext4_forget() when things
go wrong.

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