page_cache_tree_insert WARN_ON hit on 4.8+

[Dave Jones]

Hit this during a trinity run.
Kernel built from v4.8-1558-g21f54ddae449

WARNING: CPU: 0 PID: 5670 at ./include/linux/swap.h:276 page_cache_tree_insert+0x198/0x1b0
CPU: 0 PID: 5670 Comm: trinity-c6 Not tainted 4.8.0-think+ #2
 ffffc900003a3ab8 ffffffffb03dc311 0000000000000000 0000000000000000
 ffffffffb0c063d6 ffffffffb018d898 ffffc900003a3af8 ffffffffb008b550
 00000114003a3b30 ffffffffb0c063d6 0000000000000114 0000000000000000
Call Trace:
 [<ffffffffb03dc311>] dump_stack+0x6c/0x9b
 [<ffffffffb018d898>] ? page_cache_tree_insert+0x198/0x1b0
 [<ffffffffb008b550>] __warn+0x110/0x130
 [<ffffffffb008b6dc>] warn_slowpath_null+0x2c/0x40
 [<ffffffffb018d898>] page_cache_tree_insert+0x198/0x1b0
 [<ffffffffb01900c4>] __add_to_page_cache_locked+0x1a4/0x3a0
 [<ffffffffb0190379>] add_to_page_cache_lru+0x79/0x1c0
 [<ffffffffb0193456>] generic_file_read_iter+0x916/0xce0
 [<ffffffffb0232090>] do_iter_readv_writev+0x120/0x1c0
 [<ffffffffb0192b40>] ? wait_on_page_bit_killable+0x100/0x100
 [<ffffffffb0192b40>] ? wait_on_page_bit_killable+0x100/0x100
 [<ffffffffb0232e29>] do_readv_writev+0x1f9/0x2e0
 [<ffffffffb025d44e>] ? __fdget_pos+0x5e/0x70
 [<ffffffffb025d44e>] ? __fdget_pos+0x5e/0x70
 [<ffffffffb025d44e>] ? __fdget_pos+0x5e/0x70
 [<ffffffffb0232f74>] vfs_readv+0x64/0x90
 [<ffffffffb023300d>] do_readv+0x6d/0x120
 [<ffffffffb0234907>] SyS_readv+0x27/0x30
 [<ffffffffb000276f>] do_syscall_64+0x7f/0x200
 [<ffffffffb09cfa8b>] entry_SYSCALL64_slow_path+0x25/0x25