xfs: split the page fault trace event

Split the xfs_filemap_fault trace event into separate ones for read and
write faults and move them into the applicable locations.

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