procfs: prefer neater pointer error comparison

procfs: prefer neater pointer error comparison

We can compare a pointer to a known error code via PTR_ERR(ptr) == -EINVAL
or via ptr == ERR_PTR(-EINVAL) - the latter is neater and collects the
macro and constant in one, so refactor to use this form in
proc_mem_open().

Link: https://lkml.kernel.org/r/2fae1834-749a-45e1-8594-5e5979cf7103@lucifer.local
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>