BACKPORT: f2fs: sanity check log_blocks_per_seg

f2fs currently only supports 4KB block size and 2MB segment size.
Sanity check log_blocks_per_seg == 9, i.e. 2MB/4KB = (1 << 9)

Partially
(cherry-picked from commit 9a59b62fd88196844cee5fff851bee2cfd7afb6e)

f2fs: do more integrity verification for superblock
    
Do more sanity check for superblock during ->mount.
    
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Bug: 36817013
Change-Id: I0be52e54fba82083068337ceb9f7ad985a87319f
Signed-off-by: Jin Qian <jinqian@google.com>