RDMA/restrack: fix kernel-doc indicator

RDMA/restrack: fix kernel-doc indicator

Use "/**" to begin kernel-doc comments. This eliminates these
kernel-doc warnings:

Warning: include/rdma/restrack.h:123 struct member 'kref' not described in
 'rdma_restrack_entry'
Warning: include/rdma/restrack.h:123 struct member 'comp' not described in
 'rdma_restrack_entry'

(not adding missing return value kernel-doc descriptions)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260224003149.3175815-1-rdunlap@infradead.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>