tracing: uprobes trace_event interface

tracing: uprobes trace_event interface

Implements trace_event support for uprobes. In its current form it can
be used to put probes at a specified offset in a file and dump the
required registers when the code flow reaches the probed address.

The following example shows how to dump the instruction pointer and %ax
a register at the probed text address.  Here we are trying to probe
zfree in /bin/zsh

# cd /sys/kernel/debug/tracing/
# cat /proc/`pgrep  zsh`/maps | grep /bin/zsh | grep r-xp
00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
# objdump -T /bin/zsh | grep -w zfree
0000000000446420 g    DF .text  0000000000000012  Base        zfree
# echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events
# cat uprobe_events
p:uprobes/p_zsh_0x46420 /bin/zsh:0x0000000000046420
# echo 1 > events/uprobes/enable
# sleep 20
# echo 0 > events/uprobes/enable
# cat trace
# tracer: nop
#
#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
#              | |       |          |         |
             zsh-24842 [006] 258544.995456: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
             zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
             zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
             zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79

TODO: Connect a filter to a consumer.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>