[XFS] replace remaining __FUNCTION__ occurrences

__FUNCTION__ is gcc-specific, use __func__

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30775a

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>