ext2: check xattr name_len before acquiring xattr_sem in ext2_xattr_get

In ext2_xattr_get(), the code will acquire xattr_sem first, later checks
the length of xattr name_len > 255. It's unnecessarily time consuming and
also ext2_xattr_set() checks the length before other checks. So move the
check before acquiring xattr_sem to make these two functions consistent.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>