Provide missing param description for 'page_index' too.

Fixes the following W=1 kernel build warning(s):

 fs/ecryptfs/read_write.c:16: warning: Incorrect use of kernel-doc format:  * ecryptfs_write_lower
 fs/ecryptfs/read_write.c:29: warning: Function parameter or member 'ecryptfs_inode' not described in 'ecryptfs_write_lower'
 fs/ecryptfs/read_write.c:29: warning: Function parameter or member 'data' not described in 'ecryptfs_write_lower'
 fs/ecryptfs/read_write.c:29: warning: Function parameter or member 'offset' not described in 'ecryptfs_write_lower'
 fs/ecryptfs/read_write.c:29: warning: Function parameter or member 'size' not described in 'ecryptfs_write_lower'
 fs/ecryptfs/read_write.c:29: warning: expecting prototype for eCryptfs(). Prototype was for ecryptfs_write_lower() instead
 fs/ecryptfs/read_write.c:248: warning: Function parameter or member 'page_index' not described in 'ecryptfs_read_lower_page_segment'

Cc: Tyler Hicks <code@tyhicks.com>
Cc: "Michael A. Halcrow" <mahalcro@us.ibm.com>
Cc: ecryptfs@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tyler Hicks <code@tyhicks.com>