f2fs: let's avoid panic if extent_tree is not created

[ Upstream commit df9d44b645b83fffccfb4e28c1f93376585fdec8 ]

This patch avoids the below panic.

pc : __lookup_extent_tree+0xd8/0x760
lr : f2fs_do_write_data_page+0x104/0x87c
sp : ffffffc010cbb3c0
x29: ffffffc010cbb3e0 x28: 0000000000000000
x27: ffffff8803e7f020 x26: ffffff8803e7ed40
x25: ffffff8803e7f020 x24: ffffffc010cbb460
x23: ffffffc010cbb480 x22: 0000000000000000
x21: 0000000000000000 x20: ffffffff22e90900
x19: 0000000000000000 x18: ffffffc010c5d080
x17: 0000000000000000 x16: 0000000000000020
x15: ffffffdb1acdbb88 x14: ffffff888759e2b0
x13: 0000000000000000 x12: ffffff802da49000
x11: 000000000a001200 x10: ffffff8803e7ed40
x9 : ffffff8023195800 x8 : ffffff802da49078
x7 : 0000000000000001 x6 : 0000000000000000
x5 : 0000000000000006 x4 : ffffffc010cbba28
x3 : 0000000000000000 x2 : ffffffc010cbb480
x1 : 0000000000000000 x0 : ffffff8803e7ed40
Call trace:
 __lookup_extent_tree+0xd8/0x760
 f2fs_do_write_data_page+0x104/0x87c
 f2fs_write_single_data_page+0x420/0xb60
 f2fs_write_cache_pages+0x418/0xb1c
 __f2fs_write_data_pages+0x428/0x58c
 f2fs_write_data_pages+0x30/0x40
 do_writepages+0x88/0x190
 __writeback_single_inode+0x48/0x448
 writeback_sb_inodes+0x468/0x9e8
 __writeback_inodes_wb+0xb8/0x2a4
 wb_writeback+0x33c/0x740
 wb_do_writeback+0x2b4/0x400
 wb_workfn+0xe4/0x34c
 process_one_work+0x24c/0x5bc
 worker_thread+0x3e8/0xa50
 kthread+0x150/0x1b4

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>