orangefs: implement vm_ops->fault

Must retrieve size before running filemap_fault so the kernel has
an up-to-date size.

This should have been caught by xfstests generic/246, but it was masked
by orangefs_new_inode, which set i_size to PAGE_SIZE.  When nothing
caused a getattr prior to a pagefault, i_size was still PAGE_SIZE.
Since xfstests only read 10 bytes, it did not catch this bug.

When orangefs_new_inode was modified to perform a getattr instead,
i_size was set to zero, as it was a newly created file.  Then
orangefs_file_write_iter did NOT set i_size.  Instead it invalidated the
attribute cache, which should have caused the next caller to retrieve
i_size.  But the fault handler did not know it was supposed to retrieve
i_size.  So during xfstests, i_size was still zero, and filemap_fault
returned VM_FAULT_SIGBUS.

Fixes xfstests generic/452.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>