ext4: replace min/max nesting with clamp()

The clamp() macro explicitly expresses the intent of constraining a value
within bounds.Therefore, replacing max(min(a,b),c) with clamp(val, lo, hi)
can improve code readability.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>