Linux 6.19 compatibility

[xarblu]

Just a tracker for compatibility with 6.19 which currently fails to compile with (at least) the following error:

  CC [M]  fs/bcachefs/btree/init.o                                                                                 
/var/tmp/portage/sys-kernel/cachyos-kernel-6.19_rc1/work/linux-6.18/fs/bcachefs/btree/init.c:59:6: error: call to undeclared function 'mempool_init_kvmalloc_pool'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]                                                                                    
   59 |             mempool_init_kvmalloc_pool(&c->btree.bounce_pool, 1,                                           
      |             ^                                                                                              
1 error generated.                                                                                                 
make[5]: *** [/var/tmp/portage/sys-kernel/cachyos-kernel-6.19_rc1/work/linux-6.18/scripts/Makefile.build:287: fs/bcachefs/btree/init.o] Error 1                                                                                       
make[5]: *** Waiting for unfinished jobs....                                                                       

Probably isn't that high priority which is totally fine - we're just a day past RC1 after all.

[xarblu]

I think this commit should have all required changes: xarblu/linux@53161ba

Compiles fine and ktest (at least the single_device one) looks good so far aside from some new vmalloc warning: Unexpected gfp: 0x10 (__GFP_RECLAIMABLE). Fixing up to gfp: 0x402800 (GFP_NOWAIT|__GFP_ACCOUNT). Fix your code! which seems to make some tests be considered failed.

The mempool stuff needs to be vendorized in some way because it was removed in torvalds/linux@8b41fb8 - not sure what the best approach is here.

EDIT:
Well seems like they did it again and removed a required EXPORT_SYMBOL() meaning CONFIG_BCACHEFS_FS=m is broken even with these changes :/
Seems like that's the only one though as re-exporting it compiles the module just fine.