xfs: Make the symbol 'xfs_rtalloc_log_count' static

Fix the following sparse warning:

fs/xfs/libxfs/xfs_trans_resv.c:206:1: warning: symbol 'xfs_rtalloc_log_count' was not declared. Should it be static?

Fixes: b1de6fc7

 ("xfs: fix log reservation overflows when allocating large rt extents")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>