pagemap.h: move dir_pages() over there

That function was declared in a lot of filesystems to calculate
directory pages.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/exofs/dir.c
Simple merge

diff --cc fs/ext2/dir.c
Simple merge

diff --cc fs/freevxfs/vxfs_lookup.c
Simple merge

diff --cc fs/minix/dir.c
Simple merge

diff --cc fs/nilfs2/dir.c
Simple merge

diff --cc fs/qnx6/dir.c
Simple merge

diff --cc fs/sysv/dir.c
Simple merge

diff --cc include/linux/pagemap.h
Simple merge