md: Fix compilation warning

Remove the if statement around the calls to sysfs_link_rdev() to avoid
the compilation warnings:

warning: suggest braces around empty body in an ‘if’ statement

when compiling with W=1. For the call to sysfs_create_link() generating
the same warning, use the err variable to store the function result,
avoiding triggering another warning as the function is declared
as 'warn_unused_result'.

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