XArray: Fix an arithmetic error in xa_is_err

There is a math problem here which leads to a lot of static checker
warnings for me:

net/sunrpc/clnt.c:451 rpc_new_client() error: (-4096) too low for ERR_PTR

Error values are from -1 to -4095 or from 0xffffffff to 0xfffff001 in
hexadecimal.  (I am assuming a 32 bit system for simplicity).  We are
using the lowest two bits to hold some internal XArray data so the
error is shifted two spaces to the left.  0xfffff001 << 2 is 0xffffc004.
And finally we want to check that BIT(1) is set so we add 2 which gives
us 0xffffc006.

In other words, we should be checking that "entry >= 0xffffc006", but
the check is actually testing if "entry >= 0xffffc002".

Fixes: 76b4e529

 ("XArray: Permit storing 2-byte-aligned pointers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[Use xa_mk_internal() instead of changing the bracketing]
Signed-off-by: Matthew Wilcox <willy@infradead.org>