f2fs: add BUG_ON to avoid unnecessary flow

This patch adds BUG_ON instead of retrying loop.
In the case of node pages, we already got this inode page, but unlocked it.
By the fact that we don't truncate any node pages in operations, the page's
mapping should be unchangeable.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>