net: buf: Fix NET_BUF_USER_DATA_SIZE value for Bluetooth

For 32-bit architecture Bluetooth only requires 4 bytes for the user
data. There are places that store a pointer in it, so we need to make
an exception for 64-bit architecture. The code contains relevant build
asserts, so it's sufficient to set a conditional default in Kconfig
but let the range definition be simpler (unconditional).

Also simplify & fix the conditional defaults & ranges. E.g. separately
mentioning X86_64 is redundant since that option explicitly selects
the 64BIT option.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>