ext4: simplify mballoc preallocation size rounding for small files

ext4: simplify mballoc preallocation size rounding for small files

The if-else ladder in ext4_mb_normalize_request() manually rounds up
the preallocation size to the next power of two for files up to 1MB,
enumerating each step from 16KB to 1MB individually. Replace this with
a single roundup_pow_of_two() call clamped to a 16KB minimum, which
is functionally equivalent but much more concise.

Also replace raw byte constants with SZ_1M and SZ_16K from
<linux/sizes.h> for clarity, and remove the stale "XXX: should this
table be tunable?" comment that has been there since the original
mballoc code.

No functional change.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Weixie Cui <cuiweixie@gmail.com>
Link: https://patch.msgid.link/tencent_E9C5F1B2E9939B3037501FD04A7E9CF0C407@qq.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>