ext4: add cond_resched() to __ext4_find_entry()

We tested a soft lockup problem in linux 4.19 which could also
be found in linux 5.x.

When dir inode takes up a large number of blocks, and if the
directory is growing when we are searching, it's possible the
restart branch could be called many times, and the do while loop
could hold cpu a long time.

Here is the call trace in linux 4.19.

[  473.756186] Call trace:
[  473.756196]  dump_backtrace+0x0/0x198
[  473.756199]  show_stack+0x24/0x30
[  473.756205]  dump_stack+0xa4/0xcc
[  473.756210]  watchdog_timer_fn+0x300/0x3e8
[  473.756215]  __hrtimer_run_queues+0x114/0x358
[  473.756217]  hrtimer_interrupt+0x104/0x2d8
[  473.756222]  arch_timer_handler_virt+0x38/0x58
[  473.756226]  handle_percpu_devid_irq+0x90/0x248
[  473.756231]  generic_handle_irq+0x34/0x50
[  473.756234]  __handle_domain_irq+0x68/0xc0
[  473.756236]  gic_handle_irq+0x6c/0x150
[  473.756238]  el1_irq+0xb8/0x140
[  473.756286]  ext4_es_lookup_extent+0xdc/0x258 [ext4]
[  473.756310]  ext4_map_blocks+0x64/0x5c0 [ext4]
[  473.756333]  ext4_getblk+0x6c/0x1d0 [ext4]
[  473.756356]  ext4_bread_batch+0x7c/0x1f8 [ext4]
[  473.756379]  ext4_find_entry+0x124/0x3f8 [ext4]
[  473.756402]  ext4_lookup+0x8c/0x258 [ext4]
[  473.756407]  __lookup_hash+0x8c/0xe8
[  473.756411]  filename_create+0xa0/0x170
[  473.756413]  do_mkdirat+0x6c/0x140
[  473.756415]  __arm64_sys_mkdirat+0x28/0x38
[  473.756419]  el0_svc_common+0x78/0x130
[  473.756421]  el0_svc_handler+0x38/0x78
[  473.756423]  el0_svc+0x8/0xc
[  485.755156] watchdog: BUG: soft lockup - CPU#2 stuck for 22s! [tmp:5149]

Add cond_resched() to avoid soft lockup and to provide a better
system responding.

Link: https://lore.kernel.org/r/20200215080206.13293-1-luoshijie1@huawei.comSigned-off-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: stable@kernel.org

fs/ext4/namei.c

@@ -1511,6 +1511,7 @@ static struct buffer_head *__ext4_find_entry(struct inode *dir,
 		/*
 		 * We deal with the read-ahead logic here.
 		 */
+		cond_resched();
 		if (ra_ptr >= ra_max) {
 			/* Refill the readahead buffer */
 			ra_ptr = 0;