bcachefs: Fix incorrect gfp flags

bcachefs: Fix incorrect gfp flags

fixes:
00488 WARNING: CPU: 9 PID: 194 at mm/page_alloc.c:4410 __alloc_pages_noprof+0x1818/0x1888
00488 Modules linked in:
00488 CPU: 9 UID: 0 PID: 194 Comm: kworker/u66:1 Not tainted 6.11.0-rc1-ktest-g18fa10d6495f #2931
00488 Hardware name: linux,dummy-virt (DT)
00488 Workqueue: writeback wb_workfn (flush-bcachefs-2)
00488 pstate: 20001005 (nzCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
00488 pc : __alloc_pages_noprof+0x1818/0x1888
00488 lr : __alloc_pages_noprof+0x5f4/0x1888
00488 sp : ffffff80ccd8ed00
00488 x29: ffffff80ccd8ed00 x28: 0000000000000000 x27: dfffffc000000000
00488 x26: 0000000000000010 x25: 0000000000000002 x24: 0000000000000000
00488 x23: 0000000000000000 x22: 1ffffff0199b1dbe x21: ffffff80cc680900
00488 x20: 0000000000000000 x19: ffffff80ccd8eed0 x18: 0000000000000000
00488 x17: ffffff80cc58a010 x16: dfffffc000000000 x15: 1ffffff00474e518
00488 x14: 1ffffff00474e518 x13: 1ffffff00474e518 x12: ffffffb8104701b9
00488 x11: 1ffffff8104701b8 x10: ffffffb8104701b8 x9 : ffffffc08043cde8
00488 x8 : 00000047efb8fe48 x7 : ffffff80ccd8ee20 x6 : 0000000000048000
00488 x5 : 1ffffff810470138 x4 : 0000000000000050 x3 : 1ffffff0199b1d94
00488 x2 : ffffffb0199b1d94 x1 : 0000000000000001 x0 : ffffffc082387448
00488 Call trace:
00488  __alloc_pages_noprof+0x1818/0x1888
00488  new_slab+0x284/0x2f0
00488  ___slab_alloc+0x208/0x8e0
00488  __kmalloc_noprof+0x328/0x340
00488  __bch2_writepage+0x106c/0x1830
00488  write_cache_pages+0xa0/0xe8

due to __GFP_NOFAIL without allowing reclaim

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>