ext4: fix use-after-free in ext4_ext_shift_extents

hulk inclusion
category: bugfix
bugzilla: 187600, https://gitee.com/openeuler/kernel/issues/I5SV2U
CVE: NA

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If the starting position of our insert range happens to be in the hole
between the two ext4_extent_idx, because the lblk of the ext4_extent in
the previous ext4_extent_idx is always less than the start, which leads
to the "extent" variable access across the boundary, the following UAF is
triggered:

==================================================================
BUG: KASAN: use-after-free in ext4_ext_shift_extents+0x257/0x790
Read of size 4 at addr ffff88819807a008 by task fallocate/8010
CPU: 3 PID: 8010 Comm: fallocate Tainted: G            E     5.10.0+ #492
Call Trace:
 dump_stack+0x7d/0xa3
 print_address_description.constprop.0+0x1e/0x220
 kasan_report.cold+0x67/0x7f
 ext4_ext_shift_extents+0x257/0x790
 ext4_insert_range+0x5b6/0x700
 ext4_fallocate+0x39e/0x3d0
 vfs_fallocate+0x26f/0x470
 ksys_fallocate+0x3a/0...