UPSTREAM: f2fs: fix condition in __allow_reserved_blocks()

If reserve_root mount option is not assigned, __allow_reserved_blocks()
will return false, it's not correct, fix it.

Fixes: 7e65be49 ("f2fs: add reserved blocks for root user")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit e75ce117)

Bug: 440276987
Bug: 446116545
Change-Id: Ie24d0d0b1b8bd81751f2210a138a4a7bc0914a12
Signed-off-by: panchuang <panchuang@vivo.corp-partner.google.com>