ext4: fix rbtree traversal bug in ext4_mb_use_preallocated

During allocations, while looking for preallocations(PA) in the per
inode rbtree, we can't do a direct traversal of the tree because
ext4_mb_discard_group_preallocation() can paralelly mark the pa deleted
and that can cause direct traversal to skip some entries. This was
leading to a BUG_ON() being hit [1] when we missed a PA that could satisfy
our request and ultimately tried to create a new PA that would overlap
with the missed one.

To makes sure we handle that case while still keeping the performance of
the rbtree, we make use of the fact that the only pa that could possibly
overlap the original goal start is the one that satisfies the below
conditions:

  1. It must have it's logical start immediately to the left of
  (ie less than) original logical start.

  2. It must not be deleted

To find this pa we use the following traversal method:

1. Descend into the rbtree normally to find the immediate neighboring
PA. Here we keep descending irrespective of if the PA is deleted or if
it overlaps with our request etc. The goal is to find an immediately
adjacent PA.

2. If the found PA is on right of original goal, use rb_prev() to find
the left adjacent PA.

3. Check if this PA is deleted and keep moving left with rb_prev() until
a non deleted PA is found.

4. This is the PA we are looking for. Now we can check if it can satisfy
the original request and proceed accordingly.

This approach also takes care of having deleted PAs in the tree.

(While we are at it, also fix a possible overflow bug in calculating the
end of a PA)

[1] https://lore.kernel.org/linux-ext4/CA+G9fYv2FRpLqBZf34ZinR8bU2_ZRAUOjKAD3+tKRFaEQHtt8Q@mail.gmail.com/

Cc: stable@kernel.org # 6.4
Fixes: 38727786

 ("ext4: Use rbtrees to manage PAs instead of inode i_prealloc_list")
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Tested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/edd2efda6a83e6343c5ace9deea44813e71dbe20.1690045963.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>