ext2: replace BUG_ON with WARN_ON_ONCE in ext2_get_blocks

ext2: replace BUG_ON with WARN_ON_ONCE in ext2_get_blocks

If ext2_get_blocks() is called with maxblocks == 0, it currently triggers
a BUG_ON(), causing a kernel panic.

While this condition implies a logic error in the caller, a filesystem
should not crash the system due to invalid arguments.

Replace the BUG_ON() with a WARN_ON_ONCE() to provide a stack trace for
debugging, and return -EINVAL to handle the error gracefully.

Signed-off-by: Milos Nikic <nikic.milos@gmail.com>
Link: https://patch.msgid.link/20260207010617.216675-1-nikic.milos@gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>