samples/bpf: sampleip: Replace PAGE_OFFSET with _text address

Macro PAGE_OFFSET(0xffff880000000000) in sampleip_user.c is inaccurate,
for example, in aarch64 architecture, this value depends on the
CONFIG_ARM64_VA_BITS compilation configuration, this value defaults to 48,
the corresponding PAGE_OFFSET is 0xffff800000000000, if we use the value
defined in sampleip_user.c, then all KSYMs obtained by sampleip are (user)

Symbol error due to PAGE_OFFSET error:

    $ sudo ./sampleip 1
    Sampling at 99 Hertz for 1 seconds. Ctrl-C also ends.
    ADDR                KSYM                             COUNT
    0xffff80000810ceb8  (user)                           1
    0xffffb28ec880      (user)                           1
    0xffff8000080c82b8  (user)                           1
    0xffffb23fed24      (user)                           1
    0xffffb28944fc      (user)                           1
    0xffff8000084628bc  (user)                           1
    0xffffb2a935c0      (user)                           1
...