vchoi_fork

Check whether or not the shadow entry size is different from the
pagebuffer entry size.  If the sizes are different, then release the old
shadow space (using the correct size) and set the shadow page number to
0 so that H5F_update_vfd_swmr_metadata_file() will allocate new shadow
space with the right size.

Vailin says that this fixes the bug she found, where a 4096-byte buffer
allocated by H5MV_alloc() is released with H5MV_free() as if it was 8192
bytes long.