f2fs: add remap_pages as generic_file_remap_pages

This was added for all the file systems before.

See the following commit.

commit id: 0b173bc4daa8f8ec03a85abf5e47b23502ff80af

[PATCH] mm: kill vma flag VM_CAN_NONLINEAR

This patch moves actual ptes filling for non-linear file mappings
into special vma operation: ->remap_pages().

File system must implement this method to get non-linear mappings support,
if it uses filemap_fault() then generic_file_remap_pages() can be used.

Now device drivers can implement this method and obtain nonlinear vma support."

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>