ext4: make num of fast commit blocks configurable

This patch reserves a field in the jbd2 superblock for number of fast
commit blocks. When this value is non-zero, Ext4 uses this field to
set the number of fast commit blocks.

Fixes: 6866d7b3 ("ext4/jbd2: add fast commit initialization")
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201027044915.2553163-2-harshadshirwadkar@gmail.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>