bcachefs: Move the ei_flags setting to after initialization

`inode->ei_flags` setting and cleaning should be done after initialization,
otherwise the operation is invalid.

Fixes: 9ca4853b

 ("bcachefs: Fix quota support for snapshots")
Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>