libbpf-tools: migrate xfsslower to fsslower

libbpf-tools: migrate xfsslower to fsslower

This commit migrates xfsslower to a generic fsslower which supports
tracing multiple file systems. It works the same way as the original
tool except that the users are supposed to specify which file systems
to trace using -t option.

sudo ./fsslower -t ext4 -m 1
Tracing ext4 operations slower than 1 ms... Hit Ctrl-C to end.
TIME     COMM             PID     T BYTES   OFF_KB   LAT(ms) FILENAME
10:36:07 code             6896    F LL_MAX  0           2.40 state.vscdb-journal
10:36:07 code             6896    F LL_MAX  0           1.74 state.vscdb-journal
10:36:07 code             6896    F LL_MAX  0           1.78 state.vscdb

Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>