f2fs: introduce f2fs_do_tmpfile for code consistency

This patch adds f2fs_do_tmpfile to eliminate the redundant init_inode_metadata
flow.
Throught this, we can provide the consistent lock usage, e.g., fi->i_sem,  and
this will enable better debugging stuffs.

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