ext4: fix the comment describing ext4_ext_search_right()

The comment describing what ext4_ext_search_right() does is incorrect.
We return 0 in *phys when *logical is the 'largest' allocated block,
not smallest.  

Fix a few other typos while we're at it.

Cc: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>