ext4: fix bug on in ext4_es_cache_extent as ext4_split_extent_at failed

In the Linux kernel, the following vulnerability has been resolved:

ext4: fix bug on in ext4_es_cache_extent as ext4_split_extent_at failed

We got follow bug_on when run fsstress with injecting IO fault:

[130747.323114] kernel BUG at fs/ext4/extents_status.c:762!

[130747.323117] Internal error: Oops - BUG: 0 [#1] SMP

......

[130747.334329] Call trace:

[130747.334553] ext4_es_cache_extent+0x150/0x168 [ext4]

[130747.334975] ext4_cache_extents+0x64/0xe8 [ext4]

[130747.335368] ext4_find_extent+0x300/0x330 [ext4]

[130747.335759] ext4_ext_map_blocks+0x74/0x1178 [ext4]

[130747.336179] ext4_map_blocks+0x2f4/0x5f0 [ext4]

[130747.336567] ext4_mpage_readpages+0x4a8/0x7a8 [ext4]

[130747.336995] ext4_readpage+0x54/0x100 [ext4]

[130747.337359] generic_file_buffered_read+0x410/0xae8

[130747.337767] generic_file_read_iter+0x114/0x190

[130747.338152] ext4_file_read_iter+0x5c/0x140 [ext4]

[130747.338556] __vfs_read+0x11c/0x188

[130747.338851] vfs_read+0x94/0x150

[130747.339110] ksys_read+0x74/0xf0

This patch's modification is according to Jan Kara's suggestion in:

https://patchwork.ozlabs.org/project/linux-ext4/patch/[email protected]/

"I see. Now I understand your patch. Honestly, seeing how fragile is trying

to fix extent tree after split has failed in the middle, I would probably

go even further and make sure we fix the tree properly in case of ENOSPC

and EDQUOT (those are easily user triggerable). Anything else indicates a

HW problem or fs corruption so I'd rather leave the extent tree as is and

don't try to fix it (which also means we will not create overlapping

extents)."