md: raid5: Fix compilation warning

Remove the if statement around the calls to sysfs_link_rdev() to avoid
the compilation warning "suggest braces around empty body in an ‘if’
statement" when compiling with W=1.

Also fix function description comments to avoid kdoc format warnings.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Song Liu <songliubraving@fb.com>