f2fs: avoid race in between read xattr & write xattr

Thread A:					Thread B:
-f2fs_getxattr
   -lookup_all_xattrs
      -xnid = F2FS_I(inode)->i_xattr_nid;
						-f2fs_setxattr
						    -__f2fs_setxattr
						        -write_all_xattrs
						            -truncate_xattr_node
							          ...  ...
						-write_checkpoint
								  ...  ...
						-alloc_nid   <- nid reuse
          -get_node_page
              -f2fs_bug_on  <- nid != node_footer->nid

It's need a rw_sem to avoid the race

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>