btrfs: Mark btrfs_assertfail() __noreturn

Fixes a bunch of warnings including:

  vmlinux.o: warning: objtool: select_reloc_root+0x314: unreachable instruction
  vmlinux.o: warning: objtool: finish_inode_if_needed+0x15b1: unreachable instruction
  vmlinux.o: warning: objtool: get_bio_sector_nr+0x259: unreachable instruction
  vmlinux.o: warning: objtool: raid_wait_read_end_io+0xc26: unreachable instruction
  vmlinux.o: warning: objtool: raid56_parity_alloc_scrub_rbio+0x37b: unreachable instruction
  ...

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/960bd9c0c9e3cfc409ba9c35a17644b11b832956.1681342859.git.jpoimboe@kernel.org