parisc: Revise __get_user() to probe user read access

Because of the way read access support is implemented, read access
interruptions are only triggered at privilege levels 2 and 3. The
kernel executes at privilege level 0, so __get_user() never triggers
a read access interruption (code 26). Thus, it is currently possible
for user code to access a read protected address via a system call.

Fix this by probing read access rights at privilege level 3 (PRIV_USER)
and setting __gu_err to -EFAULT (-14) if access isn't allowed.

Note the cmpiclr instruction does a 32-bit compare because COND macro
doesn't work inside asm.

Signed-off-by: John David Anglin <[email protected]>
Signed-off-by: Helge Deller <[email protected]>
Cc: [email protected] # v5.12+

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