f2fs: remove unneeded disable_nat_bits()

Commit 7735730d ("f2fs: fix to propagate error from __get_meta_page()")
added disable_nat_bits() in error path of __get_nat_bitmaps(), but it's
unneeded, beause we will fail mount, we won't have chance to change nid
usage status w/o nat full/empty bitmaps.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>