ext4: make ext4_ext_get_blocks always return <= max_blocks

ext4_ext_get_blocks() returns number of blocks allocated with buffer
heads unmapped for a read from prealloc space.  This is needed so that
delayed allocation doesn't do block reservation for prealloc space since
the blocks are already resevred on disk.  Fix ext4_ext_get_blocks to not
return greater than max_blocks, since some of the code paths cannot
handle such a return value.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>