recovery: Mount block map path before checking whether it is canonical

[    4.747659] recovery: Failed to get canonical of /cache/recovery/block.map, No such file or directory
[    4.759769] recovery: Block map path @/cache/recovery/block.map not canonical, abort installation.

Change-Id: I70ee25fcb419b3802d836bc206e3154b6cff7ea3
Signed-off-by: danielml <daniel@danielml.es>