f2fs: optimize trace_f2fs_write_checkpoint with enums

This patch optimizes the tracepoint by replacing these hardcoded strings
with a new enumeration f2fs_cp_phase.

1.Defines enum f2fs_cp_phase with values for each checkpoint phase.
2.Updates trace_f2fs_write_checkpoint to accept a u16 phase argument
instead of a string pointer.
3.Uses __print_symbolic in TP_printk to convert the enum values
back to their corresponding strings for human-readable trace output.

This change reduces the storage overhead for each trace event
by replacing a variable-length string with a 2-byte integer,
while maintaining the same readable output in ftrace.

Signed-off-by: YH Lin <yhli@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>