ext4: remove unused variable in add_dirent_to_buf()

After commit 978fef9 (create __ext4_insert_dentry for dir entry
insertion), 'reclen' is not used anymore.

Signed-off-by: Guo Chao <yan@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>