slab: recognize @GFP parameter as optional in kernel-doc

slab: recognize @GFP parameter as optional in kernel-doc

Since the @GFP parameter in kmalloc_obj() etc. is now optional, change
the kernel-doc to indicate that it is optional. This avoids kernel-doc
warnings:

WARNING: include/linux/slab.h:1101 Excess function parameter 'GFP' description in 'kmalloc_obj'
WARNING: include/linux/slab.h:1113 Excess function parameter 'GFP' description in 'kmalloc_objs'
WARNING: include/linux/slab.h:1128 Excess function parameter 'GFP' description in 'kmalloc_flex'

Fixes: e19e1b480ac7 ("add default_gfp() helper macro and use it in the new *alloc_obj() helpers")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260617163125.2716279-1-rdunlap@infradead.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>