s390/cpum_sf: Simplify release of SDBs and SDBTs

s390/cpum_sf: Simplify release of SDBs and SDBTs

Free_sampling_buffer() releases the Sampling Data Buffers (SDBs)
and Sampling Data Buffer Table (SDBTs) allocated at event
initialization. Both buffers are of PAGE_SIZE bytes. Each SDBT
consists of 512 entries. The first 511 entries point to SDBs
the last entry points to a successor SDBT. The last SDBT in
the list points to the origin of all SDBTs. SDBTs do not
contain holes, that is an entry always points to a SDB.
If less than 511 SDBs have been allocation, the last entry
points to the origin SDBT.

Simplify the release of the SDBs and SDBTs, walk along the
SDBT chain, release SDBs and SDBTs and stop when reaching
the origin again.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>