f2fs: fix the wrong f2fs_bug_on condition in f2fs_do_replace_block

This f2fs_bug_on was introduced by commit 2c190504 ("f2fs: check
segment type in __f2fs_replace_block") when there were only 6 curseg types.
After commit d0b9e42a ("f2fs: introduce inmem curseg") was introduced,
the condition should be changed to checking curseg->seg_type.

Fixes: d0b9e42a ("f2fs: introduce inmem curseg")
Signed-off-by: LongPing Wei <weilongping@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>