ext4: Add allocation criteria 1.5 (CR1_5)

ext4: Add allocation criteria 1.5 (CR1_5)

CR1_5 aims to optimize allocations which can't be satisfied in CR1. The
fact that we couldn't find a group in CR1 suggests that it would be
difficult to find a continuous extent to compleltely satisfy our
allocations. So before falling to the slower CR2, in CR1.5 we
proactively trim the the preallocations so we can find a group with
(free / fragments) big enough.  This speeds up our allocation at the
cost of slightly reduced preallocation.

The patch also adds a new sysfs tunable:

* /sys/fs/ext4/<partition>/mb_cr1_5_max_trim_order

This controls how much CR1.5 can trim a request before falling to CR2.
For example, for a request of order 7 and max trim order 2, CR1.5 can
trim this upto order 5.

Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/150fdf65c8e4cc4dba71e020ce0859bcf636a5ff.1685449706.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>