f2fs: remove unnecessary f2fs_lock_op in f2fs_new_inode

This can be removed, since f2fs_alloc_nid() actually doesn't require to block
checkpoint and __f2fs_build_free_nids() is covered by nm_i->nat_tree_lock.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>