gfs2: document ip in __gfs2_holder_init kernel-doc comment

gfs2: document ip in __gfs2_holder_init kernel-doc comment

Building with W=1 reports:
Warning: fs/gfs2/glock.c:1248 function parameter 'ip' not described
in '__gfs2_holder_init'

The ip parameter was added when __gfs2_holder_init started saving the
gfs2_glock_nq_init caller's return address to gh_ip. This makes it
easier to backtrack which holder took the lock. Document @ip to silence
this warning.

Fixes: b016d9a84abd ("gfs2: Save ip from gfs2_glock_nq_init")
Signed-off-by: Sukrut Heroorkar <hsukrut3@gmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>