ext4: add new pending reservation mechanism

[ Upstream commit 1dc0aa46 ]

Add new pending reservation mechanism to help manage reserved cluster
accounting.  Its primary function is to avoid the need to read extents
from the disk when invalidating pages as a result of a truncate, punch
hole, or collapse range operation.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Stable-dep-of: 131294c3 ("ext4: fix delayed allocation bug in ext4_clu_mapped for bigalloc + inline")
Signed-off-by: Sasha Levin <sashal@kernel.org>