xfs: split write fault handling out of __xfs_filemap_fault

Only two of the callers of __xfs_filemap_fault every handle read faults.
Split the write_fault handling out of __xfs_filemap_fault so that all
callers call that directly either conditionally or unconditionally and
only leave the read fault handling in __xfs_filemap_fault.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>