ext4: convert fault handler to use vm_fault_t type

Return type of ext4_page_mkwrite and ext4_filemap_fault are
changed to use vm_fault_t type.

With this patch all the callers of block_page_mkwrite_return()
are changed to handle vm_fault_t. So converting the return type
of block_page_mkwrite_return() to vm_fault_t.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Matthew Wilcox <willy@infradead.org>