f2fs: remove i_sem lock coverage in f2fs_setxattr()

f2fs_inode.xattr_ver field was gone after commit d260081c
("f2fs: change recovery policy of xattr node block"), remove i_sem
lock coverage in f2fs_setxattr()

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