s390/tape: Move idal allocation to core functions

s390/tape: Move idal allocation to core functions

Currently tapechar_check_idalbuffer() is part of tape_char.c and is used
to ensure the idal buffer is big enough for the requested I/O and
reallocates a new one if required. The same is done in tape_std.c when a
fixed block size is set using the mtsetblk command. This is essentially
duplicate code.

The allocation of the buffer that is required for I/O can be considered
core functionality. Move the idal buffer allocation to tape_core.c,
make it generally available, and reduce code duplication.

Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>