xfs_io: fix building with musl

xfs_io: fix building with musl

The fallback in case the libc doesn't have or doesn't advertise the
existence of d_reclen in struct dirent uses d_namlen. Musl neither
advertises d_reclen nor does it have a d_namlen member.

Calculate the value for d_namlen from d_name in the fallback path.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Reviewed--by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>