f2fs: Add inline to f2fs_build_fault_attr() stub

f2fs: Add inline to f2fs_build_fault_attr() stub

When building without CONFIG_F2FS_FAULT_INJECTION, there is a warning
from each file that includes f2fs.h because the stub for
f2fs_build_fault_attr() is missing inline:

  In file included from fs/f2fs/segment.c:21:
  fs/f2fs/f2fs.h:4605:12: warning: 'f2fs_build_fault_attr' defined but not used [-Wunused-function]
   4605 | static int f2fs_build_fault_attr(struct f2fs_sb_info *sbi, unsigned long rate,
        |            ^~~~~~~~~~~~~~~~~~~~~

Add the missing inline to resolve all of the warnings for this
configuration.

Fixes: 4ed886b187f4 ("f2fs: check validation of fault attrs in f2fs_build_fault_attr()")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>