include/linux/fs.h: fix comment about struct address_space

Before commit 9c5d760b ("mm: split gfp_mask and mapping flags into
separate fields") the private_* fields of struct adrress_space were grouped
together and using "ditto" in comments describing the last fields was
correct.

With introduction of gpf_mask between private_lock and private_list "ditto"
references the wrong description.

Fix it by using the elaborate description.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>