libsframe: refactor out sframe_fre_grow_tbl

libsframe: refactor out sframe_fre_grow_tbl

Usage of a global int number_of_entries is likely unnecessary.  The same
global is used for growing the FDE tbl too, when adding FDEs.  At the
moment, however, carve out a new function to grow the FRE table, and
use a macro instead of 'number_of_entries'.

This refactoring helps provide basis for a later patch where we add
SFrame FREs in bulk instead of one at a time to the SFrame encoder
object.

Reviewed-by: Jens Remus <jremus@linux.ibm.com>
libsframe/
	* sframe.c (SFRAME_FRE_ALLOC_LEN): New definition.
	(sframe_grow_fre_tbl): New definition.
	(sframe_encoder_add_fre): Use the new function.